The heavy gimbal will change the PIDs that are found during tuning. If you tune with gimbal and then remove it, you will just have slower response than normal. If you tune without gimbal and then add gimbal, you will get oscillations, so avoid this.
The best answer is to have default PIDs in bank1, "no-gimbal" PIDs in bank2, and "yes-gimbal" PIDs in bank3.
Then set up your flight mode switch with what ever bank you need. If could even have one switch position that uses bank2 and another switch position that uses bank3.