Have you read My Empire of Dirt (http://archiveofourown.org/works/1831612/chapters/3934216) (and sequel (http://archiveofourown.org/works/2029809/chapters/4403619)) by cuccoo4cas? It's a loooong AU with first time Sam/Gabe and Dean/Cas, but in roughly equal parts (possibly more focused on the Sam/Gabe side). Heavy on the angst, happy endings to be had. :) I really enjoyed these stories.

Also for Sam/Gabe: Hold Me Close and Believe (http://meh-forget-it.livejournal.com/134419.html) by meh_forget_it: "Sam has an imaginary friend from a young age, in fact, it's his only friend. He is withdrawn and shy around anyone that isn't his dad or Dean. When he's fourteen, John comes to the conclusion Sam is ill in a way that John can't fix and has to send him to an institute for 'disturbed youth'. However, Dean isn’t entirely convinced that Lo is as imaginary as everyone assumes."

Cold as a Stone (http://archiveofourown.org/works/3226109/chapters/7024010) by greymichaela: "In the heart of Chicago, a thrift store called Reruns acts as a haven for the young, homeless, and destitute. Its owner, Gabriel Novak, is always willing to lend a hand to people in need, but Sam Winchester doesn’t seem to want anyone’s help. Gabriel is drawn to the too-skinny young man with the startling eyes, even as Sam pushes him and his charity away. When Sam is busted for prostitution, Gabriel bails him out and agrees to take custody of him for thirty days to keep him out of jail. But Sam’s pimp, the notorious and deadly Jed Prince, is looking for him, and what begins as a budding friendship soon turns to romance as Gabriel fights to keep Sam safe."

Old Dogs and New Tricks (http://archiveofourown.org/works/3322751/chapters/7262168) by maadskittlez29: "Between co-owning a bar and performing street magic in his free time, Gabriel Shurley is a witch who's pretty much got his act together. When he comes across a down on his luck familiar, Gabe offers him a place to stay until he gets back on his feet. But Sam Winchester has had more than his share of bad experiences with witches, and he's not exactly eager to add to that list. Getting him to warm up to the witch won't be a walk in the park--especially when Sam's past isn't as far behind him as he'd like." (extremely minor Dean/Castiel in this one)

For Sam/Castiel

Elements of Journalism or: How to Find Friends and Alienate Your Superiors (http://archiveofourown.org/works/1070857/chapters/2148910) by story_monger: "Sam Winchester needs a job. He's a news reporter who recently stepped down from his comfortable position with the Los Angeles Times after his mix-up in a questionable case of journalism ethics. Luckily for him, his brother and Bobby think they can snag him a job at the New Eldritch Herald, a small-town newspaper in the middle of Kansas. And they do. And Sam meets Castiel Novak: the photography editor who everyone agrees resembles the Grumpy Cat meme. Several things happen after that: Sam and Cas develop a friendship, Cas keeps photographing everything, Dean and Sam try to figure out how to be brothers again, Sam starts to realize Editor-in-Chief Michael Alef sort of hates him, and everyone partakes in a hefty dose of vigilante journalism."

Changing is Everything (http://archiveofourown.org/works/2400254/chapters/5306708) by letmegeekatyou: "Castiel’s store is full of books that have been well-loved. He isn’t interested in books that find their way easily in the world; he prefers the damaged ones. His own damage, he keeps hidden away: he keeps his shop neat and his apartment private, keeps his life orderly and small. Small until he meets Sam, the not-so-small owner of the bakery next door, that is. Sam is outgoing and happy and ignores Castiel’s “No Food or Drinks” sign. He’s got his own damage, but he’s kind, and maybe, Cas thinks, he can make room for someone else in his orderly little life."