First, there is no gasket. None, nada, zilch. It's just RTV. They straight up lied to you if that's what they said. I have provided the pictures multiple times. Go find one on the exploded engine view, I would bet a pinky finger you won't find it.
Second, not having the boost sensor break out cable hooked up would confuse the piggyback. You need to clear the CEL from when it wasn't hooked up. It will clear on its own in 3 to 4 decent drives or you can clear if you have an OBD dongle or you can pull the negative battery lead for 30 minutes (step on the brake pedal a few times after disconnect) and then reconnect but that clears EVERYTHING learned. Not a huge deal but it will take 100-150 miles to settle back in again for best performance.
Last, that sound you heard was the turbo cooling pump that runs after shutdown when the turbo exceeds a certain temp. It heading into summer and you will hear it more and for a longer time. Just like the radiator fans that run for a few minutes after shutting down the engine, this cools the turbo bearings to keep them from sludging up. In the old days, you idled the engine for a few minutes before turning off. Now they are smart enough to do it for you.