GTCC for Rental car

I am at a C school enroute to my next command currently and need a car to get to school. I am staying off base (5 miles distance) because there was no lodging on base. I received a CNA letter. Can I use the GTCC for a rental car? I have over a month left in school. I tried to contact my detailer, but no luck getting a call back as I imagine he may be busy. Who can I contact to get authorization for the rental if needed.