Thanks for posting up here and hanging out. It looks like you have an idea in that the gantry is at fault but you are approaching the situation all wrong. First off, we do not ever use a Bubble Level to level out a printer. The reason for this is that we are actually trammeling the print head to the bed of the printer. This is basically a reference of the nozzle being level to the bed. What you have measured is the gantry of the print head being level to the ground (gravity) and the bed, in this case, may still be out of level even if you did account for the offset.
So what you need to do for this printer is actually tighten the screws on the left side of the gantry so that the cantilever side does not droop as much when the print head moves across.
Using an auto bed leveling system will help as well but it will not resolve the root cause in this case.