Where you put a T piece for a gauge, how come if I just leave the T piece on with no gauge attachted I still get gull boost? I thought I'd lose it?

It depends on how bit the hole in the T piece is. You'll be loosing pressure but not a lot as the hole is so small.

It sounds like you have cut the pressure sensing line to the wastegate. If this is cut the wastegate will remain closed and the turbo (may) overboost.

This is quite dangerous and is not recommended.