In "The Gift of the Magi" it shows that no matter what the couple will always love each other. In the song "Unconditionally" Katy Perry is saying she loves someone unconditionally! When you love someone you would do anything for them. And Katy perry loves this person without a doubt and doesn't have a care in the world what anyone else thinks about them!
Unconditionally - Katy Perry - Lyrics On Screen - Clean version