See p.32 and 33 of the TF User manual to setup the G-Switch as an AUX switch to control the looper. It's fairly easy.

Your next request is harder – the looper is not a separate mode, so you cannot redefine the AUX switch settings when it is active. With some experimentation, you may be able to find compatible settings for the same switch. For example, you could perhaps define one of the switches as both Bank Down and Looper Stop. Depends on how you plan to use it.