Is it ok to enter a giveaway for a GOTY edition of a game if you already have the base game.

Just politely ask the giveaway creator in the comments.

it's up to the giveaway's creator

ok thanks

Ask the creator, if he allows it. I personally refrain from doing so (even when GA creator is ok with it), but that's just me.
Also, if you're talking about TR then I don't think it's worth it. :p

is that possible to put in steam a code of GOTY edition while you already have the base?

I read for this in the description or ask the creator as others mentionend, but even if the creator allows it, I refuse from entering...if the Base game is most of the content and the DLCs are just some small (maybe optical) addons. E.g. Tomb Raider Goty. I only own the base game....and there are really many DLCs...but I in my opinion still someone that does not own the base game should win this....so I don´t take a entry-spot on those, even if the creator allows it. If I would not own BL2, and there is would be a goty Giveaway and the Creator would allow it, I would enter, cause in this case the difference to Base is very big.....but of course this is only my frog-mind....nobody is forced to think so.

sorry for wall of text with onks stupid philosophy

There is no rule that strictly forbids you of doing so, however the giveaway creator is allowed to request a new winner if you already own at least one item of the pack given away (number of items in the pack & their value aren't relevant, it will be approved as long as the winner owns something from the package).

So in the end, it's up to the gifter. It is often specified in the giveaway description, but if it is not you can ask in the comments.

sometimes the dlc/extra stuff is worth more than the base game. it's up to the ga creator as others mentioned.