The latest injury is Gumbleton's third hamstring tear in 12 months.
"We're not going to hide away from what's happened because we want to assist him so it's no use saying it's not serious because it is," Bombers coach Matthew Knights said.
"Scott's devastated, very disappointed because he understands the club is very keen to give him a lot of opportunities at centre half-forward or high forward and he understood he would get a lot of opportunities to settle into the team.
"I said to him last night, when I went down at quarter-time, his time will come and that he has to hang tough and believe himself and there will be a time when he'll play three or four years without injury."