I hesitate to recommend a lighter spring, at least for now.
I suggest you do a lot of double action shooting and see if your hand, fingers develop more motor strength.
You might find that after a few weeks and several sessions, live and dry, that that trigger no longer seems so heavy.
I suspect you have not done a lot of DA shooting to date. If I'm wrong, I apologize.
However, I'm fully aware that GP-100s, just like Smith & Wessons, can vary from gun to gun.
Just an aside but in the FBI's revolver days using K-frame Smith & Wessons, the agency I believe wanted its agents to DA pull a trigger rapidly about 35 times in as many seconds with factory springs.