First, I am not an electrician, so this is uneducated advice.
Your 240V 50A breaker is only rated at 25A per leg. I would guess this is true for GFI's also, but I'm not sure. So, I don't think you can get 30A 120 off of one leg of a 50A breaker.
A 240V 60A GFI would allow you to run 2 x 120V 30A breakers (don't need individual GFI for those then) and probably best suit your needs. #10 for 30A, #6 for 60A
Now there's some take delight in the carriages a rolling
and others take delight in the hurling and the bowling
but I take delight in the juice of the barley
and courting pretty fair maids in the morning bright and early