Ooof. Best of luck to you. As an owner of a GL1000, they are in a different league when compare to the 750. Everyone goes on about their reliability, which isn't wrong. But it takes a LOT to get them to that point.
The good news is that any top end work is pretty simple. Heads are plentiful and any valve work is dead simple. The bad news is that any stator or clutch work means you have to pull the motor.
Also, removing the fuel tank means taking it out through the back. Rear fender off, rear tire off, rear brake off, etc.
Just wait until you get to the carbs. ABSOLUTE nightmare.
If you have any big questions, I recommend ngwclub.com. Bunch of knowledgeable individuals more than willing to help out.
(Looking back through this thread, I can shed some light on why the craigslist ad for the yellow GL1000 was so expensive. The yellow '76 was a one-year only model, and many were repainted due to lack of sales. Current estimates state a production number of 500 models, so they're pretty collectable. That, and they look dang cool!)