RoninPA is correct about the 5" GP 100's being made, Lipsey's has a line of exclusive, half-lug, 5" barreled GP100's in 327 Fed Mag, 357 Mag & 44 Special that are just beautiful…i have been trying to get my hands on the 357 Mag version for a few years without any success!
Also, RugerGuy is right on about Arizona Arms Sales, LLC in Gilbert, AZ…Mike Reber used to be a member here (Acrylictnk) and provide the members with special forum pricing on all kinds of Rugers but he has not been active since he had uses with the old software and couldn't log on any longer.
I reached out to him to try to get him to return, but I think the experience of not being able to get his account fixed left him cold….I wish he would return….always free, fast shipping and very easy to work with. You can check him out on Gunbroker under the name, Acrylictnk.
Good luck….