D&T is the best route, but I've had good luck with the Weigatinny "no drill" mounts also (replaces the plunger style front sight and the rear sight). But it doesn't appear that they make one for the 5" GP100. You might be able to call and see if they'll roll you one, otherwise, bit the bullet and have it D&T.

As an alternative to the scope, if you're interested in an open lens red-dot sight like the Docter, Burris FastFire, or Jpoint, then there's a no-drill rear sight replacement site base available from JP rifles that has treated me well. The rear sight for the GP100, Redhawk/SRH, and SBH are universal, even though Jp rifles site doesn't mention the GP100 on their item list.