I would say from reading this that the best way to see a GPS forum started is to see if there are GPS threads, which there are not.
I will say that on other motorcycle forums I frequent none of them have GPS forums, but there are GPS threads. Like Dave said, if there is enough Threads there will be a forum. I know that over on the Rounders Forum we have a GPS route database that someone volunteered to put together and it has not really taken off, which surprised us.
Yes, it is a chicken or the egg thing, but once there is enough interest the tipping point will be hit.