The only was to get it off the drywall is to cut it out.
this is the correct answer then replace with green rock. certainly more labor thatn you were looking for but if the backside of the drywall (brown paper backing) is molded and wet, which I assure you it is, then this is the only option.
If the molded wall has insulation in it you will need to remove it too. The studs if molded and wet should come out but can remain if you cn sufficently dry them out before reinstallng new drywall and moisture barrier.
for a temporary bathroom I reccomend a 5 gallon bucket, two 12" pieces of 2x4, a roll of TP, and a can of OFF. Man up, your in for some work.