Is the 10mm about the same as a 357 Mag?
10mm conversion on the GP100 would require moonclips?
I figure it would be novel to have a 10mm revolver, but a 610 from S&W can be bought, and the barrel cut down, and recrowned by a good gunsmith much faster than making a recut of the GP100 cylinder, and rebore the barrel for a 40 caliber bullet.
You may be happier with a full blown 357 mag thumper round from Buffalo Bore or Doubletap to exceed the 10mm performance figures.
That GP100 is a stout little revolver that can handle some loads, if your hand can deal with it.