My gas sensor is stuck physically, how can I fix it?
Typically that's stainless-steel mesh. I suggest an inexpensive (many of them come from Pakistan or India) set of dental picks, just hook it where it got bashed in and gently try to pull it out. They are quite useful tools to have around anyway. 



If this is a combustible gas detector, keep in mind that part of the purpose of the mesh is to make the sensor explosion-proof by cooling a flame front below the ignition temperature so if you leave a hole you may have compromised the safety.