Yes, you can use the SIM100 to monitor combustible gas detectors as long as the detector has an auxiliary contact. An example of a combustible gas detector with auxiliary contacts is the Macurco GD-1. This detector has a trouble relay, which can be used to indicate several important failure modes including a) power lost to the detector, or b) power leads or auxiliary contact wires connected to the SIM100 have been cut or corroded and fallen off.
Please see the following link for wiring diagrams and more information on the Macurco GD-1 combustible gas detector: