After a trip to the range my GP100 cylinder wouldn't close- I noticed that the ejector was sticking part way up.

After disassembling the gun and many cuss words, I found the problem to be the screw inside the cylinder that holds the ejector together had backed almost all the way out. This is the screw that you need a special tool to remove.

I ordered the tool but wanted to know what, if any, loctite to use to keep this from happening again?

Red or blue?

I'd use blue.  Be sure to degrease the threads on the screw and in the cylinder(? - not sure how it's made) before putting the Loctite on the screw threads.  It really doesn't take much.

Thanks. That's kinda what I was thinking. I guess the heat could weaken the blue locktite, but I'll give it a try.

How is the End Shake, Barrel Cylinder gap and cylinder timing? Your problem may be more than just a loose screw.