If the only problem is the glass leaking, I believe you don't need to completely disassemble the gauge. Use the outer bigger wrench to hold the outer ring from turning, and use the smaller tool to turn the inner star ring. The will access just the glass and associated gasket.
If it was me, I'd drive the car and brake to a stop with vigor from slow. No slosh in glass = low enough to remove with confidence.
If the seat where the glass sits is cracking or crumbled, you might need to take off the outer ring as well to take it off the car and clean it. I think its going to be easier to remove the glass on the car, compared to generating enough torque on something that small and weak when it's off the car. You can't put it in a vise.