[GSRICK]

I wonder what the front end feels like with the front wheel raised off the ground. Is it "notchy"? Damaged bearings?

Change your timing belts!

There are no timing belts on a Suzuki GS1000GL. That would be a Honda GL1000 or Goldwing.

What you say about getting the front end up is a good idea. It should have a center stand that will help in this. Put it on the stand and have someone sit on the passenger seat or if you have a scissor jack raise the front off the ground by jacking it up under the header, just pad it and be careful. Grab the tire and see it the is a lateral movement in the wheel bearings. Also grab the wheel and push and pull front to back, to see if there is any play in the steering bearings, but I don't know why that would have any effect on left turns only. When you turn the front end lock to lock, is it equal distance? Could you have a broken off or bent steering stop?

 

[GSRICK]

Here is a manual for your bike. http://gsarchive.bwringer.com/mtsac/~cliff/storage/gs/GS1000.pdf
PDF page 160 has your swing arm torque spec. Here is the site I got it from. BikeCliff's Website It is a treasure trove of valuable information for Suzuki GS models and other general information that applies to most bikes.