In your steam library right click on Garry's Mod and click on properties.













You should be able to see a editable text box involving launch options. In the launch options, add
"-r_emulate_gl" to the launch options and close the properties menu.













Restart your game if it's still running.
Launch any map you'd like and turn on "net_graph" if you have access to the developer console, which can be enabled in keyboard advanced options.
Garry's Mod should have a much higher FPS depending on your computer's graphics card and CPU.

You can also try using DirectX 12, which is proven to be more stable on modern computers.
Head over to the game directory, go into the lowercase "garrysmod" folder, and then go to the cfg folder until you see the autoexec.cfg file. Open it with notepad or any other file editing program you use.

























Add to the file, save it and relaunch the game if it's already running.


Add "-tickrate 33" to your launch options, but replace the number with any number lower than 66.


This is HIGHLY UNRECOMMENDED.
You can try setting your dxlevel to a older version. (Minimum DXLevel is 70)
Edit autoexec and add mat_dxlevel 81 to the file, but replace the number with any number lower than 120.
Do this if you know what you are doing.

If you liked this guide give it a thumbs up. Also, this method can work on any other source game, too.
Thanks :)

Fun Fact: OpenGL is what Minecraft: Java Edition used for rendering things.