Many manufacturers drive the pins in with a hydraulic press, so removal can be very difficult. A proper roll pin punch with a concave surface makes a big difference. Having something to properly support the gas block so that force is being applied in a straight line down the pins' axis will make life easier as well...
Update: Can't say for sure if every Mossy Tactical is this way but I can assure you, the gas block pins on mine WILL NOT come out. Others may not have this issue but mine will be going to the drill press to have them removed....hopefully they are not hardened!