Hi! Can i apply my game into my steam library? I prefer steam...i want that only because of multiplayer,its just...much better for me...you know,like the retail games...just type the code

Not really no, only games purchased through the Steam store are a part of your Steam library.

What you can do is add it as a non-steam game. This will allow you to launch it via Steam and include the Steam overlay + keep your screenshots. It will not track play time nor have any other features an actual Steam version would have such as reviews, workshop, etc.

If it's Steam multiplayer you want, then GOG supports that. See here for the announcement. GOG and Steam can now Cross-play. No idea how matchmaking will work though.
But in general, GOG games don't activate on Steam.

Okay,thanks! :)
Well,i knew about that,i mean just like...using steam is much practical,but that's okay,i can play like it is :)
Thanks you all for answers,have a nice day

This steam support article explains how you can add a non-steam game (as a shortuct) into your steam library:

support.steampowered.com/kb_article.php?ref=2219-YDJV-5557

(can't post link due to low rep due to low post-count so you have to copy and paste it into your browser)

That is not entirely true. Although there is the option to cross play, if the steam community wants it can choose to play steam only. This prevents people from GOG joining if they so choose.

Thank you for the correction :)