A few weeks ago, I participated in a "post 10 books that impacted your life" activity. Among the novels and fairytale / mythology compilations (D'aulaires' Book of Greek Myths was my childhood), I posted two nonfiction books. One was Nigella Lawson's How To Be A Domestic Goddess, which was my first real baking book: luxurious, accessible, described with Nigella's inimitable sense of humor and flavorful flare.
The other? GURPS: Basic Set (Third Edition).
GURPS stands for Generic Universal RolePlaying System, and it was the first RPG that I really latched onto. The advantage of GURPS was that it allowed for storylines in a wide variety of genres and even allowed those to cross over. Some of the terminology sticks with me, too: one of the balance factors used to, for instance, allow a Roman legionnaire and space pirate to interact was the concept of tech level. Tech level was an umbrella description for the technological/scientific advancement of a particular world (historical area / region / planet / etc). Characters from settings with a lower tech level got extra points to compensate for their handicap.
My memory of timelines is a little fuzzy, so I can't swear the small collection of old D&D books I purchased didn't pre-date GURPS, but it was definitely the first system that I took out into the world. I created characters across a wide variety of imaginary realms. I created weeks-long adventures and campaigns for friends, who soon discovered I had no poker face whatsoever and would make predictions about what was going to happen next and then watch my expression. Not fair, y'all.
I remember very clearly during the early phases of one adventure that I got a long email from one of my players explaining why she thought one of the NPCs (non-player characters - in other words, one of the cast I controlled) could not be trusted. I was so stoked by the amount of thought she put into it. It was one of my best moments as a GM (GameMaster). Heck, it was one of my best moments as a writer, which probably doesn't say much for my writing career ...
In any event, my stint with GURPS launched me into years of online roleplaying games, where I made friends, honed my writing craft in a social venue, and goofed off. I would occasionally generate writing characters with GURPS, but I eventually moved away from it because the system was *too* flexible and too close to narrative. I preferred to use the artificial strictures of RPG character generation to suggest possibilities and/or force me to think about things in a different way.
Another aspect of GURPS that has come in handy as a writer is their sourcebooks are very well researched (where relevant) and nicely thought out. I've been known to use the guidelines in GURPS: Religion for the mythos of a new world. GURPS: Faerie, with its broad overview of common fairy legends and attributes, provided me a great jumping off point for research. ... and just for fun, GURPS: Grimoire introduces one to sorcery such as the "Guns To Butter" and "Transfer Pregnancy" spells ...
I'd never suggest the sourcebooks as a substitute for research, but they offer a solid starting point, and more than that: they're geared for those trying to tell stories with the elements they feature. Shameless plug over.
I've done one more fun thing with the system more recently. I took the master skill list and, using a random number generator, selected four or five to give to character(s). Explaining this bizarre assortment, and my trying to figure out a plot that would utilize each skill, led to the story Waterways. In Waterways, immersion in the sacred waters puts a person in touch with ancestors and allows them to access past skills ... only with the city under occupation, things haven't gone quite as intended.
Waterways was fun to write, but I don't think I'm done with this concept. We'll see where it leads in the future.