Unhook the hose at the anti-ventilation plate by turning the yellow adapter fitting 1/4 turn and lift it out.
Now see if it looks clean and see if the gauge does down.
If the gauge does not go down unhook the hose off the back of the gauge and see if it goes down, If not then it's a bad gauge or clogged fitting on the back of the gauge, ect.
Have you tried tapping on the gauge yet to see if the needle in the gauge will go down?
Now with the hose pulled off at both ends, Blow air through the hose, Try to have someone else at the other end of the hose to ensure it has good air flow, Also to see if any dirt, ect, has come out of the hose.
Now you will want to try to find a air hose fitting that will let you blow air down from the anti-ventilation plate area through the lower units little hole you just drilled out, see if you get air.
If yes then reinstall the yellow fitting at the anti-ventilation plate and blow air from the lower unit hole and have someone inside the boat to see if your getting air at the end of the hose where it hooks up at the gauge, If your getting a good amount of air, Then hook the hose back up to the gauge using a wire tie to secure the hose from blowing off either from water or air pressure, Then blow air from the drive units little hole and see if the gauge moves.
What I use to blow air down from the anti-ventilation plate is another yellow fitting with a little hose hooked up to it, But You could just unhook the hose from the back of the gauge and blow back as another option, Just have someone down there by the drive to see if your getting air.
If you can not get air coming out of the little hole on the lower unit when you blow air down from the anti-ventilation plate where the yellow adapter fitting goes, Then you may half to drop the lower unit to clear out the passageway.
Good luck.
