The list of gateways is published in the contract. However, it's subject to adding and removing various gateways periodically.

Bottom line, if you're near an airport that has multiple airlines servicing it with multiple flights, it's more than likely on there.

Here's a dated list of some, don't take it as gospel. Sorry for the formatting, didn't copy-paste well;

ABQ DEN ANC DFW ATL EWR BOS FLL BWI GEG CHS HOU LAX PDX LGA PHX MDW PIT MEM RDU MIA SAC MSP SAT

CLE IAD MSY SEA CLT IAH OKC SFO CMH JAX OMA SLC COS JFK ORD STL DCA LAS ORF TPA DTW MCO PHL BDL ISP PVD BHM BNA CVG JAN MCI MDT MKE OAK PBI PNS RSW SMF BOI DAB VPS AVL CRP DAY EUG FAI HLN IND LIT MGM SAN SGF SYR SHV TLH TUS TYS