You would just need to know either the offset from the tcp of the RG2 and then add that to the RG2 TCP offset or calculate it from the wrist of the robot. You can also have the robot calculate the TCP (Assuming you are referring to a UR) by touching off on a single point in space from three different poses. This would create a new offset based on where the vacuum is.
I have RG2 gripper and a vacuum attached beside the gripper. The vacuum is offset from RG2 so , how I can calculate the TCP if the vacuum offset?