The model# for the Lyman Peep sight is: 57. If your GPR was manufactured before 1997 you will have to drill (1) hole. If it is newer you will not have to drill any and the instructions come with it for installation and it is as easy as changing the screw lenght of one tang screw and utilizing the small machine screw hole that is located between the 2 tangs screws. I have a left handed Lyman GPR with the Lyman no.57 peep sight. The above info about mounting is taken from the lit that came with the sight.
I really like mine and I took some advice from "Tundragriz" here on this sight about the front sight. He advised not to use the target globe sight due to issues with low light. I did purchase the target globe front sight from him but ...he was right and I eventually replaced it with the fiber optic I was using.
IMHO the sight is fantastic at the range and also in good bright light situations. I was preparing for a bear hunt in Maine all summer shooting at the range in good light situations. Then, one late afternoon 2 days before I left I walked out in the early evening light and found I had great difficulty seeing well enough to get a good sight picture. Worried about the conditions in Maine I changed back to the fiber optics.
I have nothing but praise for the sight itself and spent in the area of $80 for it. The nice thing is I can switch out to the peep from the fiber optics in the field with 2 screw drivers if lighting is an issue. I hope to give them a try this season in snowy conditions where light should not be a factor. If not I'll keep the fiber optics on. I found that you also can use the peeps with the fiber otics still on the gun. You can actually see them through the peep.
I was shooting 1"- 1 1/2" groups at 50yds with consistancy with the peeps. I think they also helped my groups with the FO also but that may just be due to burning powder with more regularity that I usually do!! I shot alot getting ready for that hunt.
Lyman Item# 3090112, Receiver sight - 57 GPR
If I can offer you any more help with this let me know!! Good Luck!!