If using the 'figure 8' tool isn't enough to hold the rifle, then carefully use a strong bench vise this way -
1) pad the jaws with wood, brass, or aluminum.
2) stand the rifle upright on a box to get it the correct height to reach the jaws - DON'T 'hang' the rifle from the vise.
The jaws should be on the very front of the gas cylinder.
3) tighten the jaws just enough to hold the rifle in position.
4) use a wrench on the gas plug.
And, YES use penetrating oil the day before!
Jay Kosta
Endwell NY USA