knew about it through watching lots of gmod animations (kitty0706, others that i cant really remember), portal theories, tf2 commentaries and alike even though i was probably 10-12 at the time
first source experience was probably begging my uncle to buy me portal 2 for the ps3, even though i had never even played portal 1
after that i got into tf2 by playing it on a shitty dell computer at my cousins house
was extremely interested in gmod, especially the animation aspect of it cause i wanted to become an animator when i was young. Downloaded a pirated copy of it and did posing and shit with the basic tools, just generally fucking around
had no money so made a bet with my friend (who was shit at tf2) that whoever wins the most rounds of 1v1ing as each class would buy the other 3(?) mann co keys, banking on the fact that id probably win all rounds (i did)
forgot about the bet until like a year or two later, randomly brought it up but struck a deal so that he'd get me gmod instead of the keys (barely played tf2 at the time and it was cheaper than the keys cause of sales)
played random shit like darkrp, doctor who rp, scp:cb back when it was still a gmod gamemode, military rp
didnt know shit about hl2 aside from general facts due to the aformentioned videos created by early 2010s youtube neckbeards, randomly logged onto neb and left
came back like a half year later, got into it more, eventually got the games gifted to me by some forum user and i finally played through them