User:Dingdong1232/Dingdong1232s' Team Fortress 2 in Garry's Mod and SFM
TF2 in GMOD
You can mount Team Fortress 2 into Garrys' Mod if you have the game installed (it moves files over to the GMOD folder or something like that) and this is about the experience of that.
TF2 in SFM
You can run TF2 in SFM apparently and that's about the experience of that.
How The TF2 SFM Leaks Likely Worked
It has been probably done the same way Deadlock has been done where if you have access you can invite your friends on your Steam list to have access. This explains how it gets leaked so easily and why random nobodies have it and keep leaking stuff from it, why there's different versions leaking as it kept updating. It kept updating until it released with Meet the Pyro coming out.
Deadlock is done by people from the TF2 team and it has some assets from it or references it.
Now the question who was patient zero in this whole thing? All it takes is invite his entire Steam friend list. I guess Valve was just already confident enough it could release anyways.
And this is probably also how TFC got leaked. Some guy called Deathace (you can see his modpage) leaked the TFC betas while they were playtesting, so I guess he downloaded the .exes, there was no Steam so it was on Valves' own website or FTP. So if this is true, this open invite playtest model is something they been doing for a while and for probably every game since than. They say "the modder leaked the password to Valves' website".
Repository
This is the mysterious blamer for all things leak and beta.