Alot of times toilet gurgling can be caused by a vent that is clogged or frosted over. In the winter the warm water causes some of the water to evaporate and flow up the vents (heat rises) to the roof. By the time it gets there, it cools and condenses again. This forms a small film on the inside of the pipe. After a certain number of times, if the pipe is undersized or has other debris in it from birds and squirrels (nests), it freezes over (if your in the north or have a cold winter). If your on a septic tank, you can only put so much water into the system before you get bubbling from toilets. Since every gallon of water you put into the system begins to create pressure (it has to displace the same amount of air), and the septic system may not be able to process and absorb as much water as your putting in over a certain period, you will have a high pressure inside the piping if the main vent is clogged. Washers are notorious for creating this problem on septic systems. Especially the newer high capacity washers, that can dispel high quantities of water in a shorter period of time.
The vent on the toilet, runs from the toilet drain up to the roof, and is designed to prevent any pressure buildup within the system. So if its disfunctional either because of under sizing or clogging, the toilet will bubble.