I think that's an easy fix, it can just be scripted so that if the gpd drops below 5 you have x minutes to finish the robbery. If you don't finish in that amount of time than you would receive the same message telling you there aren't enough gpd online. I would think something like 30 or 45 min would be plenty of time for a legitimate robbery to be completed.
BOOM!!!! theres a solution to an issue with a legitimate middle ground to agree on. The way i see it if you start the robbery with the 5 GPD and one logs off it could very easily be seen as combat logging, BUT if we set a time limit I.E. an hour that seems like a legit middle ground that would be difficult to abuse by either party, thus balancing it out.