I'm glad this is in the right thread. There is one kind of rubber I would never re-use.
That said, I would consider re-using glass gaskets IF they are not cracked or torn. One crack or one tear would be a re-use deal breaker for me.
Here at the Edge of the World, the sun dries out gaskets to the point where they are as brittle as saltines. These can not be revived.
The windshield gaskets I have been able to re-use are generally coated with WD-40 that I use in removing glass. I wipe the gasket with clean shop towels then soak in a solution of Dawn and hot water overnight. (Yes, I know the water cools.) Then I dry the gasket again and soak with a heavy application of Armor-All. I let it set for ½ hr or so, then dry thoroughly.
Some here aren't big fans of Armor-All and you may be one of them. It has worked for me in the past. I really don't think the brand is that important, but what it does, is. Use a functional equivalent in a similar manner.
When I have to buy a gasket, I go to Steele Rubber Products. Another source is Metro, but I have heard some complain about fit.