The term "Gen" probably arose from the term "general" as a rating used to indicate that a fan work was suitable for all audiences. It has evolved to refer to Fanfiction that doesn't focus on romantic and/or sexual relationships.
Defining exactly what is and isn't "Gen" however, can be difficult. There has generally been little tension over how fics are classified in Supernatural. Gen fics tend to be those that, like the show itself, that may have sex or passing romantic relationships, but don't have them as central to the narrative.
To what extent they can occur in a Gen fic can be contentious. Wincest is usually totally disallowed, although other slash pairings may be permissible if not the focus of a fic. A story set during the year Dean lived with Lisa could be classified as gen if their relationship is not the focus of the story.
Gen fic makes up about 38% of Supernatural fic (Supernatural Fic Link Archive: Source). While figures are difficult to find, anecdotal evidence indicates this may be much higher than in other fandoms (For example see discussion on Gen as a minority genre in this post). This may be partly attributable to the proportion of writers and readers uncomfortable with Wincest or RPS when the fandom began, which makes up the majority of the other fic in the fandom (about 39%). While some writers will only write Gen] fic, there are many in the fandom who will write stories that includes Gen, Slash and Het.
"Gen" no longer relates to the rating of a fic. Gen fic may range from G through to NC-17. NC-17 fics tend to be those with high levels of violence, or horror themes. The themes covered in Gen fic are wide - ranging including Case Fic, Curtain Fic, Hurt-Comfort, Post-Apocalypse, AUs and Crack. Pre-series stories, particularly those featuring the Wee!chesters are also popular. Gen fics may also focus on the platonic relationships between characters.
Given that Sam and Dean have a very close canonical relationship, some writers will note on their fic that there is "Wincest if you squint" e.g. even if a sexual relationship isn't explicit in a story, a reader could easily read the story as if there was. Even on the Show there have been references which blur the line - such as Zachariah's comment to Adam in 5.18 Point of No Return: You know Sam and Dean Winchester are psychotically, irrationally, erotically codependent on each other, right?
Gen fics are exceedingly rare in RPF stories.
- Gen fic tagged on delicious as part of the Supernatural Fic Link Archive
- SPN Gen fic comm
- SPN Gen Love comm a rec comm on LJ for Gen fic.
- Gen fic focusing on Castiel
- Gen Fic Exchange