Is it possible to hide game you own from giveaway list?

It should be hidden by default if i recall correctly. Otherwise you can go here and toggle it.

You can also manually filter them item 5 on the page Luk7nk4 directed you to, this can be done by clicking the eyeball icon on any running giveaways. I have had to do this myself on some dlcs and game of the years

I already went there and set "yes" to hide games I own. But it's still showing them. I already synced my account recently.

Looks like it was my mistake. I double checked "sync". It was saying last time synced 4 days ago. Then I synced again. Those giveaways disappeared. That function working as intended. My apologize.

for dlc's and autosync try to use this script