Be prepared for more answers than you can digest.
1. Don't own a GP-100, have a Security Six though. Top notch.
2. 38 Special for home defense ? There are no magic bullets or calibers, what really matters is how much you will practice to achieve proficiency. A hit with a 38 Special hurts a lot more than a miss with a .357.
3. 4" vs 6"-we will argue this till The Crack of Doom. For home use, a 6" is fine. Some argue the longer barrel gives an enemy more to grab onto. I say squeezing the trigger will take care of that.