@Wing Rider , I also have Lyman 50th. If you look at the .38 special data you will see there is a listing for 2400 with a 158gr JHP. Look at those pressures and you will see that 2400 can be reduced quite a lot from the suggested starting loads in the .357 mag data. The same is evident with .44 mag and .44 special data.
As to the accuracy with reduced loads of 2400, I can't really speak to that with any quantitative evidence backing me. I simply haven't tested this powder for accuracy with reduced charges. But I have shot a few that were below the suggested starting charge for .44 mag, and they didn't seem inaccurate during casual shooting.
I'd suggest trying a few at the starting charge of the .357 mag data using the 2400, and see how they feel to you. Bear in mind that the velocities listed in the data are from the test barrel length (I believe it's a 4" barrel for the Lyman data). So with the 6" GP100, you can expect a bit more.
As far as projectiles go, I have enjoyed the coated 158gr LSWC from SNS Casting in my .357 target loads. Which have gone from a very mild 3.8gr of WST in a .357 case, to a charge of 15.5gr with 2400
which I do NOT recommend! And this is where the Lyman 50th gets tricky, because there is no data in it for a 158gr LSWC. There is data for both 155gr and 160gr lead bullets, using 2400. And the charges for both seem at odds with each other given the 5gr difference in bullet weight. Getting data for this combo (158gr LSWC and 2400) from a reputable source proved difficult for me. However, I then found Lyman's Cast Bullet Handbook 4th Edition. This manual does have data for a 158gr LSWC, which is listed as 13.5gr maximum. The 15.5gr load was pretty scary and surely over pressure, plus I did see flattened primers and leading towards the muzzle. I backed way off after that range session. This is not an experience I have shared on this forum before, and only do so now to help you avoid getting confused but the data you do have and doing something foolish like I did. Perhaps other test data exists with a higher charge than the 13.5gr, but I have yet to find it.
Having said all that, the SNS coated bullets have performed well for me at a wide range of velocities, and so from .38 special up to that 13.5gr of 2400 in a .357 mag, should cause you no issues at all. They are also much cheaper than XTPs.