this is by far the hardest step, but i somehow managed so you can too. according to reliable sources the most efficient way to find friends is by talking to people and socializing but because you're looking at a guide on how to get your friends to play garrys mod (assuming you have any of those) i doubt you do much of any socializing so id suggest finding friends online. finding friends online tends to be easier than irl because real people are scary, and online friends wont shame you for listening to pinkerton by weezer every day on repeat for 3 - 12 hours straight. finding friends online can be done either by using social media (i.e discord or smth like twitter (i would advise against)) or considering you're already a seasoned gmod veteran trough games like csgo (i would advise against) or gmod itself (i would advise against). the reason why i think its a bad idea to join public servers on gmod is because you will tend to not find any sane people in them (depending on which you join). i wouldnt recommend csgo unless you speak russian. by far the safest place to find mildly - slightly more than mildly sane people is tf2. the way you actually make people into your friend on said multiplayer games is to either interact with ingame chat/voice chat or top score in the game and hope people friend you (ill let you guess which one is more viable). if you do manage to complete this step it only gets easier after you do, you've done it once, you can do it again. to add to that, if you find one friend you can get exposed to his/her friend group afterwards (most of the time)

this one is pretty easy, make sure they either have the game already (if they do they would probably play the game with you if you asked anyways) or you could convince them to buy it, or if you dont mind throwing away money at other people (if so paypal me 60 bucks, i wanna buy elden ring) you could buy it for them, usually when you buy it for them they feel obligated to play it because you spent money on them, if they dont give a ♥♥♥♥ and tell you to ♥♥♥♥ off or smth like that go back to step 1 and try again.

this part is probably the second hardest part, especially if they dont agree right away, my advice is to find an interest they have that overlaps with the things you could do in gmod i.e playing ttt or gamemodes or trying out various addons. just remember to not overdo it because then you will seem annoying and they might not want to play with you at all. id also suggest showing them clips/memes of the game possibly baiting them into being like "hey!! this looks fun maybe we can do some of this funny ♥♥♥♥!!!" because everyone knows even spending 8 hours a day cleaning ♥♥♥♥ off sidewalks is fun with friends (i think). another way could be (i wouldnt recommend this though) begging them continuously until they get so frustrated they end up trying it anyways. the reason why i dont think this works is because most of the time they will find you annoying, and because of how people work will refuse to enjoy the game in any way (worst case scenario) so id suggest agains't this. eventually if you have enough of a friendgroup established (could just be you and another guy) you can have other people look onto you guys playing the game and naturally would want to join in because if they see you guys doing something fun they'll most likely feel like doing that aswell, yknow blob eats up more people and gets endlessly bigger. another way ive seen to convince people to play games with you is being humble about it, yknow like them pretentious ancient greek men, you don't really shove it in their face and just mostly keep to yourself about it, although still sometimes exposing the game to them subtly and over time they'll (if all goes well) warm up to the game and either ask you if you wanna play with them or you could throw them the question, nothing pushy just "hey wanna play gmod with me??"

congratulations, you did it! you have friends now and they agreed to play sandbox with you, this gave you the motivation to fix your life and your life will now become like all those hustlers on social media telling you how to make your life better promise (usually dont deliver) you will now finally be loved by your family and will no longer struggle to lift those grocery bags your mom always asks you to help bring home and you will gain muscles and get like a girlfriend or something whatever your life goals are.

(this is you now)