Why is Garry´s Mod so hard to come by while trading?

It's not been bundled and is a good game

There wasn't too many discounts or bundles for Garry's mod so people are not holding it to get profit :) the best shot is actually steam store and waiting for sale

It's VAC game, can't be purchased for trade anymore

Well I can't give you a 100% accurate reason (because there are all sorts of possible variables) but MOST of the time (not all but most) games that pop up on, say, this site are ones that are currently on an amazingly good sale somewhere or another or are packaged together in a deal, aka humble bundle style. So unless G-mod gets a massive sale and/or get's bundled then chances are you wont see it much, if ever. You will one day. It will just take a potentially long time. Also leveling up on here grants you access to more levels of giveaways so, you may just not be able to see any giveaways on here due to your level. Does that help at all?

"VAC and Game Ban enabled games can either be bought for your own account, or gifted immediately. They can no longer be saved to your inventory for trading purposes. If you directly gift a game to someone who gets permanently banned, you won’t be able to gift that game again."
Source: Here

I have it and I think many other traders still have it, but as proximus said, your offer isn't good.

:o You already own Garry's Mod! What kind of trolling is this :p

garrys mod of trolling :)

Garry mod is tagged as VAC enabled game, and new copies is not tradeable anymore (just like another VAC game anymore)
As a trader, I'm also looking for tradeable copies of Garry Mod since it can sold well.. :3

It's because it can't be stored in your inventory anymore. I'm currently selling some copies on kinguin though. There's a trick, where you can buy the game and send it to your e-mail, then copy the link that's inside the e-mail and give it to the buyer.