I think joint marketing efforts and collaborations would enable you to build stronger bons with your contractor. Also, during the sessions you can exchange brilliant ideas that will enable both of you to grow more
Since you’re a sub, I’m presuming you’re providing, but this is actually too broad; we need to know exactly what the contractor is providing and what business they’re in.
Having said that, “you refer me, I refer you” is usually how I feel. “Official” collaborations that share marketing materials irritate me. primarily because, in my experience, people become too comfortable with this shit and start producing terrible job, and I don’t enjoy being locked to anyone.
Since the GC is referring you, I think I would give them a gift for the holidays. Tickets to a sporting event or something like. I wouldn’t take direct bribes. In the event that you come across someone in need of a GC, ask them for a stack of business cards.