Being able to use the GMOD camera is very useful, both for making event pictures and for GM sits made by players.
The GMOD Camera comes by default on the staff job, and is in the 6 slot.
It is good to have people pose before you use the camera, and it might be useful to shoot the ground a bit to show them where to go.
Equip the camera and left click. Screenshots will save into your Steam screenshots/into your files at
C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod\screenshots
One problem you may notice with the camera is that chat and other UI icon show up. To disable these and make the screenshots better, take out the camera, and hold down C. In the top left of your screen, click on Drawing (it might be under the FPS counter). In the popup, uncheck Draw HUD.
Chat and other UI icons will not be shown anymore, and you can keep clicking with the camera to take pictures! When done, you can check the Draw HUD button again.