Flushing the toilet in your bathroom should in no way affect the other drains in the bathroom, including your shower, unless a problem exists in your house's plumbing. Gurgling means air does not move as it should through the drain pipes, which may lead to the entrance of sewer gasses into your home.
Your toilet and shower have separate drains, but the drain pipes the two plumbing fixtures drain into eventually connect. The drain pipes in your house also connect to vent pipes, which run straight up and down. These pipes eventually go through the roof of your house, where they not only release the sewer gasses in the drain pipes but also introduce fresh air into the plumbing system. Every time you wash water down the drain, the water carries air with it, but the vent pipes introduce more air, keeping the drains flowing smoothly.
When you have drains gurgling in your house, you need to get onto the roof so you can inspect the vent pipes. Check each pipe for any damage; the pipes may crack or break apart and fall into themselves, blocking the flow of air. Birds may build nests on the pipe openings as well and debris can fall into the pipes to create a blockage. You will need to shine a flashlight down the vent pipes so you can look for clogs.
Some debris will sit near the top of the vent pipes, making the extraction process something you can do with your hands. If you see the debris sits further down the vent pipes, use a tool such as a narrow retriever to pull the debris out of the vent pipe. Irrigate the vent pipes with your garden hose to knock the debris loose. Use a high pressure attachment on the hose to increase your chances of dislodging the debris.
If you cannot clear the vent pipes yourself, or if you do not see anything wrong and yet the gurgling problem continues, you need to contact a local plumber for professional help. Plumbers will send fake smoke through the vent pipes so they can located a clog or other problems easily. A plumber also has a long auger or snake that he will use to clear obstructions that sit deep in the vent pipe.