Several operational issues
Gyrotrons are big. They need large magnets in order to work, so you won't be fitting one down the borehole. That means you will have to direct the microwave energy down the borehole.
The gyrotrons produce mm-wave energy. That means having to pipe it around using waveguide. Waveguide is quite lossy at those frequencies, so you need to do trickery like quasi-optics to keep losses low. This needs finely machined and polished waveguide pipe, which is very expensive.
When you get the energy to the target, you have to deal with the vaporized material. This is one of the problems with beam weapons. You puff off a cloud of vapor and immediately your beam starts heating the vapor instead of the target. So you have to get rid of the vapor, some 20km away.
Finally, you have your hole and can start producing steam. You can't run that steam directly into a turbine. It has a lot of nasty silica and acids in it. These tend to clog up the pipes. The local geothermal plant periodically cleans the scale off and sends it off for reprocessing because it has significant amounts of gold in it. So yo you need to have a heat exchanger. Only then can you produce power.
Given these difficulties, this may work sometime, but don't expect it soon.