I'm having to do a repair on my avenge-x as well. The only videos I've found are by Shooter's Den on YouTube. There are two of them that show a complete tear down of the rifle. The only problem is that it's in Hindi, but if you get the english translation in the captions, they are very helpful.
As for the pressure gage, just unscrew it with a wrench. There is a "dowty" seal behind it (a metal ring with an o-ring inside it). You might have to replace that. What might also be helpful to you is the schematics for the rifle that are on Pyramyd Air's website.
What I need to know is how far down the pressure gage needs to be tightened. The regulator gage on mine seems to be screwed in flush, but the pressure gage is not.