Try relieving a little bit of the tension on the hammer spring yourself. That usually makes a revolver trigger tons better right from the get go. Make sure to test fire it after so doing to make sure that you still have enough hammer strike energy going on to reliably ignite all primers every time.
Ruger GP-100's are very rugged and simply designed revolvers with far fewer small moving parts than, say, a Python and a few other revolver designs so any decent gunsmith worthy of the name should be able to polish up the bearing surfaces and replace a couple of springs in it for well under $100.00.
Good luck!
No, never on the hammer spring. It has nothing to do with a SA trigger pull. Only getting rid of creep and a lighter trigger spring is needed.
I actually use over power Wolfe hammer springs of 26# to get 1-1/2# triggers on Ruger's and BFR's. Standard hammer springs are 22# to 23#.