If you are just going
TBE then there is no need for a boost gauge as you won't be changing the stock boost control; i.e.- the stock turbo, wastegate actuator and wastegate solenoid etc will all still be looking for the same boost targets, it's just that the
TBE will allow the turbo to reach the boost levels earlier in the rev range due to less restriction in the system.
The only reason to get a boost gauge imo is if you change the stock boost control system (ebc/manual boost tee) or if you are curious to know or because fast and furious lol