Pardon me while I crack my knuckles and dig into this one.....
GI lighting: On Gorgar, you're not going to find any GI related anything on the power supply, it totally bypasses it. It goes straight from the transformer, through the fuse card, and off to the GI lights in the playfield/backbox. If you have no GI lights, it's either a busted wire or a bad fuse. Under the power supply you'll see a card with three fuses and a pass-thru connector on it. The left-most fuse if for your GI lights, should be a 20A fuse. Check for voltage at the fuse and the pass-thru (yellow wire and yellow/white wire).
Sound: Your sound card runs on a totally independent power supply and CPU chip that are mounted on the sound card. On the top left of the sound card is a momentary push button. This is the self-test for it, and pressing this button should start a loop of various sounds/speech. It only needs to have the speaker, volume and power connectors..... it will run even with the main CPU and driver boards removed from the machine. Also, when you power up the machine it will normally make a buzz/hum noise for about 1-2 seconds. If neither of these work, check both fuses on the sound board (A/C power input).
No-Boot: If you're not booting up at all, first thing is first... check the LED's. There are two LED's on the CPU board. Do they just turn on? Or do they blink once, then turn on? Sounds minor, but it really helps track down a problem. Also, the "1496 01" you were seeing on the displays previously is "audit mode". Means your batteries, or battery holder, are no good. It's reverting back to base settings, and clearing out the settings/audits.
This link has fantastic schematics, which are the same as Gorgar for the boards (wiring is of course different.)
http://www.firepowerpinball.com/
IPDB also has the full Gorgar manuals and schematics available too
(Caution! 73mb download if you click this link)
http://mirror2.ipdb.org/files/1062/Gorgar_Manual.pdf
If you need to get the boards repaired, that's right in my area of speciality. I don't know why, but about 80% of the boardsets I get in are for Gorgar. It's really seeing a lot of interest lately as I'm finding out. Personally I find the theme a bit confusing, but love the gameplay. The theme is that you're the Conan looking dude, and you're fighting against Gorgar.
I can vouch for the guy at Mayfair, visited the place in person to pick up a backglass for Black Knight. Really nice guy, spent a while talking to him about stuff. Apparently Williams re-ran a number of 80's era backglasses around 1991/1992 or so due to the terrible flaking problems. They used higher quality inks, thicker inks, and also clear-coated them too. The one I picked up looked phenomenal. Shipping is spendy, but he does pack them up properly.
-Hans