The Gambit Fantasy Club is currently managed by BiggDoggProblem, and I believe it may be the most popular and best-managed club on the site.
The array of thematic gambit tournaments is excellent, and they tend to fill fast.
It is also good in the competitive sense. I have played over 300 games in club tournaments, and my club rating seems to stay 150-200 points below my regular RHP rating.
Either that, or I am an easy mark for Gambiteers!
Disclaimer: Paul Leggett did not receive financial remuneration for this endorsement. As usual, he is paid in rating points taken from our other members.
I second Paul's recommendation. Although not a gambit player myself, one must always be prepared to meet a gambit player. The GFC is an excellent way to hone one's skills.
EDIT: Endorsement disclaimer: I have not received any gift points from either Paul or BDP.