"Pyramid is the monthly PDF magazine for serious gamers, created by serious gamers. Each month, Pyramid delivers articles about a specific theme, from modern-day superheroes to post-apocalyptic gaming to magic on the battlefield. Most of Pyramid's articles are either generic, suitable for your game system of choice, or designed for GURPS, one of the best choices for serious gamers for over 20 years. We also feature humor, ready-to-print props and tools, and other diversions; we may be serious but we're fun, too."
The toughest thing about writing for GURPS is getting a handle on the WYSIWTG (what-you-see-is-what-you-get) format. The guideline index is here.
Steve Jackson Games also publishes GURPS material with Warehouse 23, modules in short PDF format. Details for that is here. And if you're feeling so inclined check out the vid of Editors Sean Punch and Steven March along with some of Pyramid's most prolific submitters.