Why is the pool pressure gauge so high?

Depending on the type of filter it needs to be cleaned or backwashed.

Cartridge filter: Hose off. If pressure does not go down buy new cartridges

Sand filter: Backwash, if pressure does not go down replace sand

DE filter: Backwash, if pressure does not go down remove element from tank and hose off and or acid wash.

Also... always make sure you gauge is working