ok, so first off have you already tried restarting your computer? if you only have 1 session of gatherbuddy, there's a chance it could be running in the background, a system restart will usually resolve this issue.
if your still getting it, log on to the shop, and make sure your paid up, its not often it will say SESSION LIMIT if not paid but it could happen. if your paid up, the only thing left is that somewhere, if you use more then 1 machine, or share a session with someone else. somewhere theres a copy of gatherbuddy running taking your session.
if all else fails, then send a pm to bossland and he will get to it as soon as possible.
Hope this helps, CodenameGamma