I like the previous post. The short answer is NO, gas checks are not a must.
Hodgdon strongly recommends them because checks do a good job at preventing leading. If a customer is not having leading problems, they are less apt to speak badly about Hodgdon powders.
So you can skip the gc's and play around a bit until you get a satisfactory load. Or you can use the gc's and your odds of finding satisfactory loads may be better. Or so I understand, I've never used a gc in my life.
I've been shooting Lil' Gun and cast bullets for years. 13 grains works perfect for my 158 grain and 200 grain cast bullets (both .357 magnum). I get some leading with the 158 grainer but never enough to feel like I have to clean it. Both bullets exceed the 1100 fps from my 6.5" Ruger, and exceed 1500 fps from my M92 24" bbl levergun.
In fact, with both the above bullets, I actually soften the bullets by annealing them (they're 100% wheel-weight) to reduce the leading.
The 158 grain load is not a full-house magnum load, however the 200 grain load is. The pressures of the 200 grain load are probably up there, but the long bearing surface and slow speed of the bullet help mitigate the high pressures. In fact, if you want to get full power from a plain cast bullet, I generally recommend going with heavy-for-caliber bullets for this reason. You can get the full ft-lbs but at a reduced velocity.