Legit to give away special edition if giveaway shows standard edition?

Yes, you can always provide more than what the giveaway describes.

Edit: Unless the special edition and standard edition have completely different app id on Steam.

Has to be same app id else it will still show up as not redeemed for the winner.

Exactly like Zomby2D said, when the gift is an upgrade it is accepted. But in those occasions where the regular and special editions have different ID codes, the giveaway should coincide with its content. Otherwise, if the winner marks as accepted and activates something other than what was defined, it will be flagged as a non-activated win (resulting in being barred from some SGTools GA's until corrected).

If needed, you can just send a ticket to Support and they will change the GA to the special edition (unless you want it to be a Christmas surprise, in which case carry on :)