They both use TDMA, but TETRA uses 4 timeslots per frame and GSM uses 8.
GSM is a cellular telephony standard and all calls are full duplex one to one. TETRA is principally used as trunked PMR and offers Group calls (one to many half duplex) as well as individual calls (one to one full or half duplex) and telephony (full duplex).
TETRA has a much faster call set up (less than 300 mS) and has stronger encryption. It also has the option of end to end encryption (i.e. encrypted through the fixed infrastructure as well as on the radio interface).