Can enter any giveaway, even if I own the game or if I don't own base game for DLC

Can you name a few examples?

The Elven Legacy addons don't show up as DLC's like other games if that's what you're talking about.

Yeah, these giveaways appears to link to Steam "bundles" comprised solely of the one game advertized for some reason, so it's possible to enter for anyone regardless of them owning the game.

Sam & Max: The Devil's Playhouse:
https://www.steamgifts.com/giveaway/vbgWq/sam-max-the-devils-playhouse
http://store.steampowered.com/sub/4172/

Dungeon of the Endless™ - Crystal Edition
https://www.steamgifts.com/giveaway/dQoyv/dungeon-of-the-endless-crystal-edition
http://store.steampowered.com/sub/54904/

Edit 1: Why do these bundles exist? One reason I'm aware of is to hide the user reviews score from the potential customer, since those do not show up on the bundle page. Reported by Jim Sterling some time ago (I forget where).

Edit 2. Sterling's game was NBA 2K11. The game is no longer available, but the one-game $19.99 "bundle" that used to contain it is amusingly up for sale.

View attached image.

Elven Legacy is the DLC I was talking about that I just gaveaway. Other games I can enter even though I own them are Kholat, Magicka DLC, Hoard Complete, etc. I also noticed I can enter for DLC for Payday even though I don't own Payday.

This is not something that SG can do anything about, it has to do with how the steam API reports ownership of games. Anything that does not get reported (which includes most but not all DLC) will be treated as "not owned", and any pack that contains something that you don't own are available for you to join.
Also, some DLC gets treated like full games by steam (in terms of how they're reported). You can join those as well, without owning the game. Speaking of which, don't do public giveaway for the Quake expansions, they fall under this category ;)