I just figured that the GP100's should have the same barrel diameter -- or at least close to one another.
Yeah, you would think. If it's all the way down the length of the barrel, it's probably not because of a tight spot, I wouldn't think. What sort of loads are you running? Mild or pretty hot? And with what powders? The action series bullets are pretty hard and you have to hit them with a good amount of pressure to be sure that they obturate (is that the right word?) so that you get a good seal to prevent the hot gases from going up the sides of the projectile.
Without slugging the barrel and taking a reading with a micrometer, it's hard to tell what is up. If you really aren't equipped to slug the barrel, you could still try a couple of things. First, I know that the Ruger GP revolvers are very nice and Ruger is a good manufacturer of revolvers. Even if you slug the barrel, it MAY not tell you what you need to know.
Now, I'm going to go assuming. Assuming that you do have a "standard" diameter barrel and .358 is what you should be using, you could try a couple of things. Order some of the softer bullets from MBC and see what they do using the loads that you've been using. I haven't had any leading problems at all, but I don't load anything mild. I load all of my stuff on the hotter side.
The other thing that you could do is order a couple of sample packs in 1/1000 larger and 1/1000 smaller and see if either of those do it for you.
As a side note, I don't think that getting a bullet to obturate is a matter of velocity, but rather a matter of pressure. What I usually do to make sure that I have adequate pressure to avoid leading is to figure the velocity that I want to obtain and then use the fastest powder that I can accomplish it with without having to run maximum loads all of the time. Everything usually runs more smoothly and cleaner when you run powders in their upper pressure ranges.
Good luck.