Does anyone know where I can get a scope mount for my gp100 with 5inch barrel that I don't have to drill and tap I have found mounts for the 4&6 inch barrel that mount to the screw holes on the gun but can't find one for the 5inch

The 5" GP-100 is not a regularly manufactured product by Ruger. I would check on the 2 Ruger forums. I am sure you will find a solution or a definite no there.

Any way some sort or pic / weaver rail could be (gunsmith) installed? That would be the direction I'd head.

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.