I would like to know whether a 3-wire gas detector can be compatible with 2-wire External Termination panel connected to a 2-wire Anaglog Input Module in PLC
Yes it can be compatible if both the module and detector are zero based, most are. The transmitter should be expecting the PLC to be connected between it's output and -24 VDC.
Post your transmitter and PLC IO model nos and someone will check it.
