Jimmy leaking fuel from front of tank

2 weeks ago, had the fuel pump replaced on my daughters 1995 Jimmy. Its a Vortec 6. Since she is 19 you can imagine filling the tank doesn't happen often. She called the other day and said that she had filled it up, and it was leaking gas. She took it back to the service center that replaced the fuel pump for an answer. They said that she probably had a seal give out on the gas tank. They could either replace it (for a cost) or she shouldn't fill it past 1/2 full because at that point it stops leaking
Upon my inspection later at home. I had her keep filling with gas until the leak started again. Right between 1/2 and 3/4 full with the Jimmy parked on an incline nose down, we got a pretty much constant stream of gas coming off the front of the tank somewhere.
When I parked it nose up on the incline, or level, it stopped. I'm thinking the service center may have missed a gasket or something when they installed the fuel pump. Especially since this had no fuel leak at all before the pump was replaced.