I'm a fan (and you should be too) because...
Nothing sounds like Gavin. Trying to describe his music to friends is impossible - a string of adjectives (bluesy, rocky, poetic) that scratch the surface but can't explain the way 15 seconds of a song can.
There aren't enough contemporary artists that use piano in their music - it gives a song a very well-rounded sound and I think it encourages the listener to focus on the instruments more instead of just the vocals (which are all most pop singers have to rely on).
Real instruments (including piano!) plus beautiful, poetic, sincere lyrics give Gavin's music an ethreal, organic sound and I LOVE IT. I try to eat organic food and buy clothes made with organic materials... why should my music be any different?