I used to have a Genco Rifle Gallery which is of the same vintage.

The entire gun assembly comes off and the entire lower assembly will come out in one piece after disconnecting the wiring too. You should replace the selenium rectifier with a modern Bridge rectifier, and keep an eye on the condition of the connectors. Replace any wires that have dried out with cracking insulation.

It'll be a project, but you'll have a very cool gun game when you're done!

You're on the right track with the glass. Remove the screws & trim and CAREFULLY slide it out so the paint on the backside doesn't get scratched. From my experience, leave the glass alone for now... If something happens to it, you'll probably never find another.

The Rifle Gallery has a different type cabinet front. The coin door was on the right side, and to remove that gun mechanism, you'd open the coin door and remove a wing nut that would enable you to swing it up away from you (it was hinged on the left side). You'd disconnect the wiring, unscrew the hinge and safety chain and the entire works comes off.

Here's a link to Clay's website that has some good information about changing the rectifier and basic EM tips...

http://www.pinrepair.com/em/index1.htm#emparts