When I flush the toilet I can hear a loud gurgling sound from the bathtub drain.
It could be that the p-trap is dry. It also can be from a blocked vent, poor wet venting, too shallow p-trap, or the drainage grade is too steep, that causes suction to empty the trap when flush.
It could also be that a house or apartment that was originally plumbed, then additional drainage load or cloth washer was added with out proper provision hence the drained p-trap/gurgling noise/sewer gas.
How about if someone upstairs in my house has a bubble bath, the bubbles appear in the toilet a floor below it lol
It suggests that the building have poor plumbing that doesn't have adequate hydrolic jump distance from the stack, and it also means that there is a blockage in the building drain just below the stack & within the hydrolic jump zone.