Garry Achievement

So my friend has the garry achievement for gmod, and I’ve read other threads and wiki’s and it claims that its impossible to get it. He claims that he got it from a jailbreak server from a hacker, is this true??
Link to the dude’s achievements for gmod:

wow dude, this is really rare! i can do you a favour too if you like bro :wink:

Anyone who googles a certain group of words that leads them to a link to a certain program can just give themselves achievements. I could give myself that achievement right now if I cared to.

He could’ve gotten it legitimately, but he could’ve also not.

There was an exploit 1 or 2 years ago that allowed users to change their steamID sent to the server and therefore giving people the achievement for it. Here’s a video of me doing it at the time [video][/video]

Can I have an explanation for this? 4 Garrys joined my server when this was popular. I know it doesn’t work anymore, I’m just interested in how it’s done.

Most people use steam achievement manager