Only 2 steps:
Step 1. Subscribe to a crosshair on the workshop. I like this one: https://steamcommunity.com/sharedfiles/filedetails/?id=1129353979, it looks great and works well with all the weapons on Monolith without blocking anything from view. Or you can find your own by searching HERE.
Step 2. Once you've subscribed to a crosshair, launch gmod and copy paste
hud_draw_fixed_reticle 1into console to enable your new crosshair!
That's it!To disable the crosshair paste
hud_draw_fixed_reticle 0into console. To change or remove your crosshair completely just unsubscribe from your selected crosshair on the workshop and/or subscribe to a different one.