The pipe I'm talking to came with the antenna. My bracket is definitely overbuilt for the antenna, but adequate. I'll try to remember to get a picture next time I go up there, or grab a photo with a zoom lens when I get home even.
In the GP6 manual
they call the bottom of the antenna the 'Power Feeding Section' and it fits down into the 'Mount Support Pipe' which comes with the antenna. Then a small bolt locks it in place. Even if you tighten up this bolt snugly, the antenna still wiggles around in the 'Mount Support Pipe' quite a bit. I'm tempted to put a couple wraps of electrical tape around the bottom of the 'Power Feeding Section' to make it a closer fit.