At some point after June 2nd, the widely-used "GlueLibrary" add-on for Garry's mod was altered so that when you make your first input on loading into a map, the game plays max volume noises and presents a fullscreen "goatse," the infamous 2000s shock image of an anonymous man's spread anus. For the love of God, don't try image searching for it in a new tab, it's really upsetting.
This story first came to our attention via content creator ThafNine on Twitter, cautioning players that the mod's creator "lost his goddamn mind" and inserted the surprise into all of his add-ons. The gmod subreddit is also abuzz over the creator's malfeasance. What makes the issue so thorny is that GlueLibrary is the basis of other add-ons, and downstream works have also been infected.
IMPORTANT PSA:on gmod right now, a popular addon creator lost his goddamn mind and has altered all of his addons to add a jumpscare of goatse when you open the game. i am not fucking joking, delete any "GlueLibrary" addon you have immediatelyJune 3, 2022
What's more, it seems that a, ahem, "Trollface Playermodel" add-on has gotten a similar, but separate treatment, with the game displaying different explicit images and loud audio of slurs in the voice of SpongeBob Squarepants, who is a friend to all and would never say such things. Additionally, another Trollface mod also seems to be infected with a similar, but less upsetting prank that hijacks parts of the UI with a chibi anime catgirl. Try saying that last sentence to someone you love.
even_more_images_with_the_recent_virus_source from r/gmod
Many players' assumption is that the creators were hacked, but text in the .lua files of GlueLibrary addons seems to indicate that this was a deliberate prank, with the files containing a message cursing out Gabe Newell, gmod creator Garry Newman, and unidentified steamworks moderators. My best guess would be that the Trollface mods' situation is a copycat prank in response to GlueLibrary.
This thread by user Spacy =WL= on Steam outlines how to check if your game was affected, as well as a discussion of how to clean up one's installation. At a glance, if you've used View Extension, Action Extension, or Ambient Occlusion add-ons for Garry's Mod, you may want to do a full reinstall of the game. Spacy's thread also contains links to clean reuploads of some of the affected projects.
For my money though, the sheer amount of copycat pranks tells me it might be safer to play Garry's Mod without add-ons for a while, at least until this wave of issues dies down or is addressed.