The GHT sits in the same building complex of Avanti Derma. It's a twin-tower building, the west tower being the hotel, the east the office building.
Levels C1 and C2 (C for "Commercial") interconnect the two buildings, and you will find an ATM in C1, in the east tower.
I'm not sure if they will exchange USD, almost positive that they won't take Euros. However, there is a money exchange place ("Casa de Cambio" in Spanish) at the east tower entrance, also on level C1. They will execute a fair deal for USD; the hotel won't.
When you check-in, you can ask them to open a voucher to cover your expenses, and you can pay in bulk at the end of your visit.
If you have any further questions, please get in touch with Diana, our patient coordinator.