There was no fear of rebellion in the sixties.
I think the assassinations played a part in the GCA, but I think the right of a lot of people to be armed while vocal antagonists of government forces in the 1960s caused the government to put an end to that situation, and like I said California did it one year earlier by outlawing previously legal open carry as well as making it illegal to protest or picket while armed.
As for having rebels under control?
The government is not as all powerful as people presume, as you need several average tax payers to pay the wages of every single government employee. When everyone is a government employee you have communism, and it doesn't tend to work, and if you want enough tax dollars to accomplish much you have to limit the number of government employees which themselves do not directly add to the Gross Domestic Product or wealth of the nation, but consume large amounts of wealth. Government depends on a much larger private sector to function.
If you turn big military armaments against the civilians the economy tends to crumble and the domestic funding for government then goes down the toilet. (Which then leaves them dependent on foreign assistance for anything that lasts longer than a short time) Using extreme violence also results in counter violence, which is why even most authoritarian governments in the world do not crush with direct force when they can make people just disappear instead without instigating violence.
Nevermind the political fallout or risk of some of your government forces switching sides when asked to do such things.So you can't really bomb your own people into compliance, so how exactly did they have it under control? They used coercion, manipulation, infiltration, and various force multipliers to maximize what they could accomplish with the resources they had.
The freedom of information act and declassified materials show things that would have been quite far fetched conspiracy theories at the time were in fact being done regularly.
The government was in fact infiltrating most groups and movements, while combating Soviet and Communist influence around the world.
Even the structure and methods of the early drug cartels stem from methods of funding counter revolutionaries in Asia, Africa, and especially South America without needing American tax dollars at the time. They were funding anti communist forces through drugs they knew were primarily going to be sold in the USA and Europe. It was widespread, some famous stories were made about some of it.
This is because communism was the will of the people in most of the third world, and only didn't come to dominate the world because the Capitalist societies fought it hard in their foreign policy.
Many of these things were all being done at this time.
We got infamous things like MKUltra, a favorite of conspiracy nuts because the government was in fact investigating ways to use mind control as well as improve interrogations.
Then we see the FBI was infiltrating many of the big movements of the 1960s. When you look into the internal conflicts of many of the groups, you often find they were instigated and even more often encouraged and escalated by comments and actions of the informants and agents. They were infiltrating many radical groups, then using those positions to turn the organizations against themselves and each other in addition to gathering intel.
After WW2 a lot of the veterans came back and didn't put up with the poor treatment workers had been dealing with for years. They demanded better treatment, formed and grew many Unions, and dealt with assassins and strike breakers that were routinely hired to intimidate, attack, and eliminate the organizers and thorns in the side of big business. It was a very hard struggle to get the 8 hour day or a 40 hour week normalized. We hear a lot about the civil rights movement, but our government curriculum doesn't encourage being as informed about the labor movement that proceeded it. Jimmy Hoffa and the teamsters are probably the biggest portion of it you will hear about.
These efforts and those behind them however were viewed with as much contempt as was held for communists by powerful and wealthy people seeing their bottom lines impacted, and as both stem from organized labor, they viewed and encouraged others to view them as communists encouraging communist policies.
We even celebrate Labor day on a different day than most of the rest of the world even though their date is based on events in Chicago, while our own government celebrates that holiday in an entirely different part of the year and tries to distance itself from that day, distance labor from comradery with other labor movements of the world, and maintain distance from a day which is often a rallying call of Communists and Socialists.)
Then comes the black civil rights movement, militant blacks, war and a draft. While the youth were in many areas abandoning tradition. A drug the government prohibited was becoming mainstream. The draft was making many feel the government was against them.
Birth control was letting women have widespread sex outside of marriage without fear of becoming a social outcast and unmarried mother. Sex is a big motivator, particularly in the younger demographic, and they no longer needed to conform to society or the lifestyle of their parents to enjoy that either.
The youth were thus a poorly controlled wild card that could join and become the foot soldiers of any other movement.
No the government was definitely at risk. All you needed was the Soviet Union to get in and arm all the youngsters, or provide training and ideology and you could have very quickly had a strong rebellion and civil war.
If some of these groups combined forces the government could also find them significantly more formidable, and some people like Martin Luther King actually started to combine the much wider and more mainstream labor movement and the black rights movement and anti war sentiment right before he was assassinated.
Imagine that combined force with the armed black militant mentality.
Bobby Kennedy had worked as chief Counsel of the United States Senate Select Committee on Improper Activities in Labor and Management on the side of labor workers being abused, and created connections with minority leaders of the time.
Martin Luther King Jr was assassinated in April, Bobby Kennedy a big ally of both the civil rights movement and labor movement and against Vietnam in June, and the GCA was passed in July, and took effect in October.
Both had been promoters of non violence and kept their supporters in check fairly well, with Bobby even reducing how much rioting and violence happened after King's assassination, but there was a lot of prominent people in those movements more in favor of armed action to fill the void when they were gone.
There was also the South where most blacks actually lived, and a large portion of white Southerners that didn't want to be forced to integrate and racially mix but were being forced to by the rest of the nation. There was already a civil war with that part of the country before over related issues, and the national guard was even being used to force integration because the local law enforcement and government sided more with segregation.
So you had military force being used to enforce policies contrary to the wishes of local populations and law enforcement.
During a serious Cold War when a strong enemy of the United States would help and assist any faction that would undermine the nation.
Government was scared and decided to remove the RKBA. They did, it ceased to be a right and became a privilege you can retain if you stay in good favor.
However before the time of computer databases it was impractical to go after the individual citizen and consumer on such a wide scale, and much more practical to deal with and restrict and keep track of and hold to specific transfer standards their sources in commerce.
The media was much better controlled back then, limited to a small number of networks, and the general public was informed according to their wishes, which did not deviate too radically from those of the government.
What was taboo to even say on air included far more than today.
Of course government wouldn't tell you there is a risk of rebellion, that is one of the biggest ways of preventing it.
When you tell people many people are ready to take action it encourages others that wouldn't but agree with the cause to also take action. So the last thing you do in preventing a rebellion is tell people there is a risk of it unless you are very confident in your ability to turn most of the population against them and their cause.