If I'm using this sentence should it be "an one" or "a one"?

Give a one-of-a-kind gift...

I believe you should use an instead of a previous to the word one. This is because one's first letter starts with a vowel.

You should write:

a one-of-a-kind gift.

"one" starts with a "w" sound. It is all about the sound not the letter.

Read more about using a and an.