The second is grammatically correct when writing. The first is technically incorrect but in conversations sounds a lot more natural than the second, but you would say "Who did you give it to?" Instead of "whom". If you wanted it to sound natural and also be grammatically correct, try a verb that has the correct connotation of "to give to someone" for the situation. For example, you might write, "Who did you gift?", or "Who was the recepient?"