ProPTT2 have a SIP-GW to integrate external equipments using SIP. They can communication with ProPTT2 clients.
The typical cases are ROIPGW, SIP phone and emergency call.
SIP-GW is a client for PBX. There are two mode. (Caller mode and Callee mode)
SIP-GW has a SIP server. So it is not required IP-PBX.
There is no PBX and no SIP server. It would be 1:1 mode.
Please refer ro SIP-GW Guide