This is a simple guide on how to play Gmod with your friends:
1. Start a new game in Gmod.
2. Choose any map you want to play on with your friend
3. At the top right of the screen there should be a green button that says "single player" that means you are playing by yourself to change that just click it and select how many players you want in the game.
4. Once you have loaded into the map you chose hold shift and tab at the same time on your keyboard doing that should bring up the friends menu.
5. Find a friend you want to play and right click on their name and select "invite to play"
6. Once your friend has loaded into the game just have fun!
I hope this guide was helpful and worked for some of you out there!