STEP JUAN:
You must get hamachi. So look it up on your internet browser and it should be the first link.

STEP TWO:
Setting up a VPN (Virtually Private Network) is very simple.

First: Run Hamachi
Two: Create a account
Three: Create a network

STEP THREE:
So after setting up your VPN/network, you/friend(s) can join a selected network. So what info you must tell them is the name of the network and password (CASE SENSITIVE) Once that is done and it shows another name in the network, you can move on to the next step

STEP FOUR:
Once you have completed STEP THREE, you can head into Garry's Mod and open up your console, if you haven't set up your console just go into Options>Keyboard>Advanced>Enable Developer Console. You can activate the console by pressing and holding down the "tilda" key (the button above "TAB" and to the left of "1") First you/ friend start a game up like so: Start New Game>Any map>DON'T TICK LOCAL SERVER>Start Game. Then wait until you/friend are spawned into the map. Then type the following into the console: connect "What ever the IPV4 is from Hamachi and paste it into these quotations"

So what to do and what not to do. To make it easier just disable all your addons or just make a pack of the addons you have mounted and tell your buddy about the pack and mount the addons.
So just don't start a server with 1000+ plus addons and tell your friends to join, like c'mon..

So this was my first guide and I hope i helped the Steam Community out. Please don't be to harsh :D