Hey Sig, welcome to DRTV, GP100 is such a versatile pistol, that for range ammo, Magtech, Georgia Arms, any decent priced range ammo will work for target ammo also.
I use the Remington 38spl MC, not the LRN, the LRN's seem a bit smoky, but not really an issue.
.357, is getting pricey, I try to keep target time with those to a minimum, just test drive different brands, to see if accuracy/groups change. At SD distances, it doesn't really factor as much of an issue.
As far as SD ammo, for 38+P, I carry Golden Saber in my snub. Or Winchester 158gr JHP's.
For my 357, I carry Remington or Federal, Winchester used to make a 110gr JHP, but they are hard to find. They even have some others up to 180 gr.
It's a 357 Magnum...Any decent top shelf ammo manufacturer will make an adequate SD ammo. Same as 38 or the +P's.
Practice with what shoots best in your GP100, and what your comfortable with. Full house magnum loads, are exactly that, the +P's are a close second, and 38 ammo has come a long way in technology and ballistic performance.
3-5 consistent shots with a good 38 rd. Is better than misses with full bore Magnum loads....Plus the deafness will go away faster.

Good shooting.