I'm trying to figure out a tool to reach the gauge without removing the boiler
Well I hope you don't think about it too much. I have NOT removed any gages from Andreja but I have from the Anita and that certainly did not involve removing the boiler. The gage(s) come out the front of the machine and are held in place by a u-clamp. The u-clamp, in turn, is held in place by the nut securing the pressure line to the gage. IIRC, a 1/2" or 12 mm wrench does the trick.
Now, yours may be different because the Andreja, like the other QM machines, has undergone lots of improvements from initial production. In that case, some pics of your machine from the front and side may be helpful for future readers of this post.