to thank is a verb. You would have to change it to thankful, which is an adjective. You are describing your feelings about the gift.

I am really thankful for the gift. I'm really thankful for the gift. Thankful, Thank You and Thanks are all different ways to express gratitude

Dear Minhthi1810 I haven't heard any native people said the sentence which was like you And "Thank" is a verb, you have to say " I thank you" but not" I am thank you"( or sth like that) But in the formal English, they usually drop the subject and I think you should correct like this: "Thanks (or thank you) for you gift." The way to improve: I think you should always find out if the word is a verb, noun or a adjective before writing a sentence. Claire

to thank is a verb. I think u could say I really appreciate your gift.