In simplest terms, because the GI is AC (sinusoidal, current flows in two directions) a Triac is used. A regular transistor or SCR is basically constructed with diodes, so it can be used as a simple DC switch. A triac is preferred in AC circuits because it allows current to flow in both directions. The triac is just a solid state switch substituting what can be done with a mechanical relay. I have noticed that in early solid state pinball, we tend to see relays used for GI switching in Williams and Gottlieb games, and triacs often used in Ballys.
A possible replacement: http://www.marcospecialties.com/pinball-parts/BT138-600E
I have been advised the following will work so this may help anyone else with this issue.
"I would use Jaycar ZX-7149 which is actually a BT-137-600V-8A Triac because it has an isolated tab meaning you don't need to use any insulating washers to mount the part - much easier."
You need to get the pin outs of new and old triacs so you can solder the wires in correctly. You can also drill a hole in the metal plate and screw the new triac in as it is isolated.