Need a lot more information on the build/parts to even begin to answer the question.

Also, the rifle is gas operated, so it this your first time behind the trigger on a high power auto loader?

If you are getting blasted in the face with gas, then it coming from between the upper receiver and the charging handle, and it originating from the back of the carrier at the bolt tail section, with maybe a bit more gas being released out the back due to the rifle slightly over functioning as well.

So lets work it backwards,

Start with a heaver buffer if this is a telescoping stock unit to try to slow down the unlock,

Next if you have an extra B/C, give it a try since it may have a tighter bolt tail to carrier channel section to reduce some of the carrier chamber section normal leakage.
Note: you have other areas that will normally leak from the gas system as well, such as the tube to key, and even past the rings, but most of these gases end up going out the ejection port instead since they are directed forward towards the barrel.

Lastly if needed, you can buy and install a gas buster charging handle so the gas is more blown out of the ejection port instead of the charging handle to receiver gap, or just break out a tube of RTV in black to install a seal on your existing handle to resolve the problem as well.
http://www.03designgroup.com/technotes/do-it-yourself-gas-buster-charging-handle

As for gas block, if you had a bad enough leak there to feel it in the face, the gun would not be cycling correctly.