How to open and use the Graphics Card on a IMAC

As for access, no iMac capable of running Catalina or Mojave has a graphics cad. All the video hardware is soldered to and integral with the logic board. No slots.

If you want access to graphics hardware via software you need to use to an external graphics processor unit (eGPU). eGPU requires a 2017 or newer iMac:

Use an external graphics processor with your Mac - Apple Support

Just check that your iMac meets or exceeds the Recommended System Requirements for the specific game(s) you want to play. If your iMac only meets the Minimum System Requirements expect to lower the quality of the graphics significantly which will usually make the game look ugly and you may still experience a poor gaming experience.

To find the hardware specifications of your Mac just click on the Apple menu and select "About this Mac".

Then just play the game. GPU switching is only an issue with computers that have multiple GPUs. For example, some 15" Macbook Pros with both integrated and discrete graphics or Macs with an external eGPU attached. Since the 2017 5K iMacs only have a single dGPU that GPU is always active.

If you are curious on which GPU(s) your computer sees and how much they are currently being used open Activity Monitor and go to Window -> GPU History. (For web browsing there is almost no activity, so launch a game in a window (not full screen so you can see both the game and the GPU History) to watch how much activity picks up.)

You should always check that the game (or any software actually) meets the Recommended System Requirements (or at the very least the Minimum System Requirements) before purchasing the game or other software. Most times you cannot get a refund on the software purchase (although Steam does have a very short period where it is possible to refund the game).

Never assume your computer is "more than capable". Always check first. Some seemingly simple games can have very extreme system requirements.

There's nothing to open or use. The GPU runs on its own.