A friend of mine bought Gary's mod via Steam and asked me to buy the game too, so we can play together via Steam. Is Gary's mod worth the investment if you have friends online to play with?
Garry's Mod is a massive sandbox that offers extreme flexibility since you can add-in weapons, NPCs, maps, vehicles, and other elements from virtually every game out there. I was hesitant for a while since there is no "game" per se. You have to go in and create your own scenarios which can be somewhat less fulfilling since you already know what lies ahead. However, your partner will not know, and if they set up the battles scenario you will not know. So, in the end, it can balance out.
I use Garry's Mod as a leap off point to play in Half-Life 2. This gives me unlimited weapons possibilities in a game that is challenging. I have gathered add on maps and weapons from Wolfenstein 3D, Minecraft, Counter-Strike Global Offensive, and scores of others.
If you are creative and enjoy virtual insanity in gaming, it will be quite a bit of fun for you and anyone you play with.