How do I add CS:S/TF2 content as an add on?

I own both games and would like to know how to transfer the content into add-on form.

It should already be there.

Unless you didn’t actually buy it. :what:

Extensions->Game content, then check CS:S and TF2, and reset Gmod.
If for some reason that doesn’t work, buy them, and it will.

I bought it, but I want to transfer it to a friend, how would I do so?

They buy it.

Gift it to them through steam

You cannot transfer the game, that would be warez. Just buy the game for him.

Login with your account on his PC, download the content for CSS and TF2, log into his account and start gmod, and the content should be able to load. I’m not sure if this is allowed though.