Do I need to have a Source Engine pre-buying G-Mod?

To run G-Mod, do I need to have a Source engine game before-hand like Half Life 2 or something?

Yes, HL2 and/or CS:S are recommended.

Technically all you need to do is install Team Fortress 2, but as Reyjr43 stated, HL2 and/or CS:S are pretty important if you want to join most servers. If all you will do is single player, then you might not need them.

Half Life 2 is not required as such seeing as all the HL2 content near enough comes with the base SDK.

CS:S will do you just fine.

Agreeing with Orange, you would likely need CS:S to play most servers, as most of them use the content.

CS:S and EP2 will do the trick.
Plus download TF2, it’s free anyways.

the SDK is free since TF2 is, so gmod can be downloaded standalone now i think

You’ll be fine with only TF2, but you should really look into picking up CSS and the Half-Life episodes (or Episode 2 at the very least), as a huge number of addons depend on content from them.

I thought that TF2 won’t count towards GMod because it’s free.
Yet i don’t know because i fucking bought TF2 in 2009.

In order to play it you need to have a source engine game.HL2 and CS:S are good to go.

With the new Update that is soon to come out you wouldn’t require a different Engine before hand, Am I correct?