Lots of us are gifted, but our kids are definitely the focus on these boards.
I also concur that the odds are high that most of us are highly gifted, but as adults we now channel that into our children/careers/etc. My guess is that your best online forums might be to look for intellectual discussions about specifically what interests you.
If you look around, you might find lots of HG adults in your own community. My children's playgroups alone are filled with highly intelligent adults who went through GATE and all that when they were in the school systems. While it would be in poor taste to proclaim to each other that "we're gifted," it can be readily apparent as you chat with people. We've had amazing talks on all sorts of "gifted" topics just by getting the ball rolling with conversation starters.
I hope you find what you're looking for!