Things required to make Gmod work

I’ve been wanting to get Gmod for some time now but I heard that you have to have source
engines to make it work. Can someone please list some of the source engines required?

TF2, HL2, CS:S.
I’d recommend any of these, mostly CS:S and HL2.
If you want more content (Jalopy car and such), also buy the HL2 EP2, or just buy the Orange Box instead.

And in HL2 do you get the scientist character? (no reason for scientist, just if it is then i know what others come with it)

You get all the HL2 content when you get Gmod AND another Source Engine game from this list:
-Team Fortress 2 (free),
-Counter Strike Source,
-Day of Defeat: Source,
-Half Life 2 Episode 1,
-Half Life 2 Episode 2,
-Alien Swarm (debatable, free, doesn’t provide gmod content),
-Left 4 Dead, (debatable)
-Left 4 Dead 2 (debatable)

Basically buy the valve complete pack and you’ll get all the source content you’ll ever need.