I have a 75 GL1000 that took almost a year to get back to running condition. It had sat for 18 years before I got it.
I have also restored an 81 GL1100 that was in pieces and had not run in 9 years.
Biggest issues are the same as the they are for the SOHC4's, the carburetors get clogged with old gas and it takes a lot of work to get them completely clean. I had to take the GL1000 carbs apart 5 times before I got them right The last time I had them apart I boiled the throttle bodies for twenty minutes that finally got it clean.
The other issue is the Brakes these bikes have disks front and rear and every one I have worked on the brakes need to come completely apart and all the seals and lines replaced. The rear caliper on the 1000 was interesting as it has two pistons that press from both sides.
In the end it was all worth it the 75 GL1000 handles good runs really smooth and goes like hell.
Remember, even though it is a horizontally opposed boxer engine it is still classified as a single overhead cam four.
Peter