I have a 20 amp breaker that is powering a portion of my pool house. I am having a problem with the circuit breaker popping without anything plugged in to the outlets. I also have a GFI outlet which is my first outlet in the string and that is not popping first. Any idea on what I need to check to isolate the problem. I already replaced the 20 amp breaker so I know that was not the problem.

Disconnect the feed at the GFI , if the GFI is bad, then the circuit breaker should clear. If the CB continues to trip, then there is a short somewhere between the breaker and the GFI.