I must assume that your reference to GSM means interfacing with cellular phone network. You can expect a four channel GSM gateway to have four SIM card slots, one for each of its channels. Inside the gateway are four independent cellular radios, typically sharing only an antenna, ethernet connection, management portal and power supply.
Your cellular service provider might be able and/or willing to put your four cellular telephone numbers into one hunt group. Otherwise, you might have to program the first phone number to Call-Forward-on-Busy to the second phone number, second to third and third to fourth. These manual call forwarding orders will likely incur per-minute charges when used.
Likewise, Asterisk treats these four SIP channels independently. It is up to you to bundle them together as an inbound and/or outbound trunk group.