It sounds like improper or obstructed venting. When a lot of water is dumped down the shower drain it completely fills the pipe. As it travels down, air must be added behind the water or a vacuum will form. This vacuum is sucking air in from the toilet causing the gurgling noise. Conversely, when the toilet is flushed, the same probably happens to the shower drain but is harder to hear or see. This can also cause the toilet to drain slower than it should. It will probably require a plumber to figure out where the vent needs to be modified or repaired.

Regards