Leveling

Nothing, it is charity in this regard.
Levels are awarded for making giveaways. The FAQ can give you more detail about it.

https://www.steamgifts.com/discussion/cQfkP/how-to-level-up
https://www.steamgifts.com/discussion/tInLS/anybody-know-how-to-level-up
https://www.steamgifts.com/discussion/CQrgG/how-to-level-up
https://www.steamgifts.com/discussion/zAoDo/how-to-level-up
https://www.steamgifts.com/discussion/PIe1A/how-to-level-up
https://www.steamgifts.com/discussion/4ppSa/how-to-get-p-and-how-to-level-up
https://www.steamgifts.com/discussion/YL93q/how-to-level-up-here
https://www.steamgifts.com/discussion/vDPKZ/how-to-level-up
https://www.steamgifts.com/discussion/bQPDK/how-to-level-up
https://www.steamgifts.com/discussion/vbBoL/does-anybody-know-how-to-level-up
https://www.steamgifts.com/discussion/5tpLn/how-to-level-up
https://www.steamgifts.com/discussion/SB9gE/how-to-level-up
https://www.steamgifts.com/discussion/OeIsY/how-to-level-up
https://www.steamgifts.com/discussion/iNNCE/how-to-level-up
https://www.steamgifts.com/discussion/J2bpy/i-am-new-and-would-like-to-know-how-to-level-up
https://www.steamgifts.com/discussion/2mTdx/how-to-level-up
https://www.steamgifts.com/discussion/dfeqA/how-to-level-
https://www.steamgifts.com/discussion/bfU04/how-to-level-up
https://www.steamgifts.com/discussion/efHt6/do-you-level-up-by-putting-games-up-for-giveaway

Edit: In most cases searching the discussion board helps a lot: https://www.steamgifts.com/discussions/search?q=level

Ty so much ,last question
can i gift an games which ive got in my steam inventory or only codes?

SG is filled with people who love to give games away without getting better from it in any way :)