From Fan History Wiki
Gen is short for "general fiction". It is generally used to indicate fanfiction without romantic pairings or that don't feature romance as a central theme.
Historical definitionsThe following definition dates to 2001 in the Star Wars fandom:
Short for 'general'. Conflicting use. Slash writers usually use this term to indicate fanfic suitable for all ages, but it can also be used to refer to fanfic with heterosexual adult relationships. The following definition dates to January 2001 and the site Bad Fanfic! No Biscuit!:
(shortened from "General") The opposite of slash. Extrapolating from above, we see that gen can be defined as either:Fiction involving sexual/intimate relationships between opposite-sex (i.e. heterosexual) couples
Fiction involving sexual/intimate relationships between two characters who do have such a relationship on the show.
- Gen/Het/Slash: categories of fan fiction, used in a wide variety of fandoms. "Gen" is short for "General" and refers to any and all stories that aren't centered on sexual or romantic relationships. "Het" refers to a story centered on a heterosexual relationship, though it's a less common term than the other two. "Slash" refers to a story centered on a same-sex pairing. It originates from Star Trek stories featuring the Kirk/Spock relationship, but applies to female/female as well as male/male pairings. Slash is hard to do well. It's the redheaded stepchild of fan fiction, and is banned from certain lists. Check for clearance before you post. 
- gen: short for general, may denote non-NC17 
"Gen" or general is the term for all stories that aren't slash or NC-17. 
fan fiction suitable for general audiences which does not contain explicit sex, rape or torture, or excessive amounts of obscene language. However, gen fic does occasionally include graphic violence. The following definition dates to August 29, 2005 in the TelevisionFic fandom:
General: Also known as "Gen." These designated fics are general in nature and tend to not focus on any ship, but rather on the show and the characters as a whole. Generally used for episodic television. If a story centered around a new vampire coming to town and the Scooby Gang et all fighting him off and did not focus on any romantic pairing ("Buffy the Vampire Slayer"), then this would be a General fic.
- Gen - A story with a heterosexual pairing. Sometimes used to refer to a story centered around a m/m or f/f friendship, as opposed to a sexual/erotic relationship. 
- gen: A story without any particular pairing. A story that is more focused on plots or individual characters than romantic entanglements.
gen: Short for 'general', denoting fan fiction of a general nature and usually safe for all age groups to read. The following definition is was written by Jane Leavell and updated in June 2006:
GEN: A story or fanzine fit for general audiences, in the original usage, and therefore lacking in explicit sex of any kind. With the widespread popularity of "slash," the meaning is changing , and "gen" is often used to mean "may have porn, but no same sex stuff." Some fen are attempting to use "het" for stories that involve heterosexual explicit sex, thus leaving "gen" as "no worse than what you see on network TV."
- Gen = fic that isn't romantic or sexual, involves no pairings 
The use of this term predates 1985.
This section needs more information.
- dzfic_genslash - a Dead Zone gen community
- quantumleap100 - a Quantum Leap gen and slash community
- tolkiengen - a Lord of the Rings gen community