Try searching Delicious. There aren't a lot of long fics with Gabriel as a major character instead of a plot device, but there are some very nice one-shots.

This has a lot of one-shots in a single 'verse, and it's funny, even if it's not quite what you're looking for.
http://goldenusagi.livejournal.com/tag/fic%3A%20spn%20-%20banter%27verse

If you like heavy-duty Norse mythology, check out Gatharr Edda, part of a series of two brief fics, here:
http://archiveofourown.org/works/137454

As a character study, I personally recommend Not The Fall That Kills, in which Gabriel meets his vessel:
http://bjewelled.livejournal.com/91204.html

This crack!fic casts the angels as a Heavenly family:
http://bellatemple.livejournal.com/423418.html
And this AU casts them as an Earthly family:
http://pyrebi.livejournal.com/33652.html

Here, Gabriel falls out of Heaven and falls in with the pagans:
http://community.livejournal.com/herostratic/54042.html#cutid1

In this three-part fic, Gabriel gets grounded and is forced to ride along on Sam and Dean's eternal road trip:
http://community.livejournal.com/spn_gabriel/213493.html

God plays dice with the universe and Kali DMs (okay, hardly any Gabriel, but it's really funny):
http://ksock.livejournal.com/23175.html

If I might self-rec, I have an AU retelling of Mystery Spot from Gabriel's POV here:
http://rokhal.livejournal.com/4457.html#cutid1

I do have one long fic, but it's WIP. It's mostly finished, though.
This long, ongoing AU has awesome plotty crack. Dean puts on Death's ring, and unlike in the show, it gives him sinister wings with giant spikes and cool powers. Of course Gabriel gets involved. It's an epic and engaging Team Free Will fic.
http://alchemyalice.livejournal.com/tag/on%20the%20wings%20of%20war