Hello
Just recently had issues with leaky gas (escapes straight out the top of the mag) in both of my mags for my Swiss Arms P1911, same as Remington TAC/RAC, Tangfolio Witness et al ; and thought i'd share the fix ;

Got one of these to remove the valve : RC-Aluminium-Alloy-Ball-End-Remover, It could be stronger but it does the job.

Sometimes the valve vibrates loose, or doesn't come fully tightened, so try that first.

If not remove the valve and there's a small o ring in the hole, sometimes there's a small washer too, but mine only had the o-ring.
The o-ring size is 7-1 which is 7mm INTERNAL diameter and 1mm CROSS Section (thickness) of the ring. These can be obtained fairly inexpensively, I got 10 for a couple of quid.

Place the o-ring on the valve and screw in loosely to position it, remove the valve and check the 0-ring is in place then fully tighten down the valve

Add gas, no leaks ? Job done.

If you suffer from loosening due to vibration you can also add a little PTFE tape to the valve threads prior to tightening.

Hope that helps, can't live without a blast on my 1911 !