If your GSM Wifi Hotspot has an Ethernet port you could connect X1 (WAN) there and it should work; that would be simplest.
If NOT then you will need to put W0 (Wireless) into Wireless Bridge Mode:
However, in both cases, you will need to ensure the correct ports within GSM Wifi hotspot router are open for VPNs:
IP protocol 50 for ESP
IP protocol 51 for AH
UDP port 500 for IKE Phase 1 and Phase 2 negotiations
UDP port 4500 is used as well if NAT-T is enabled
That's to allow the remote site to initiate the connection, you would need to publish/NAT inbound those ports to the W0 interface if you wanted to initiate from the central office to the remote site.
I would suggest testing this before sending any equipment out as the little routers inside those hotspots are often troublesome. If it works please post back with the make and model.