Contrary to popular belief, original Gamma World and d20 Future aren't the only ways to do science fiction with a D&D-like system. While those have been out of print for years, a small independent publisher has been quietly and steadily producing Stars Without Number and a stream of supplements in print and PDF. It recently won a design award, the judges of which should make an AD&D 2e player sit up and take notice: David 'Zeb' Cook, Robert Kuntz, Steve Marsh, Sandy Petersen, and Dr. Dennis Sustare.
SWN is a game loosely set in a Traveller-inspired universe, using AD&D-era mechanics that will feel familiar to a group that's coming from AD&D 2e. It's actually based more directly on Red Box D&D so you won't find "THAC0" mentioned by name, but the math underlying the system is identical and all your mental habits from AD&D will translate just fine to SWN.
The setting is loose, and is more a collection of genre tropes that you as GM can slot together. It starts with the usual pile of random-or-pick tables for tech level, population, type, biosphere, etc. to get a new planet's state of affairs off the ground, but continues with a "tagging" concept that gives you an additional layer of story hooks and interactions with other setting details just by adding a few evocative words from an extensive set of lists:
The final step in world creation, and perhaps the most important, lies in assigning “tags” to the world. Tags are brief conceptual tropes that set the world off from planets of otherwise similar population and characteristics. You can either pick from the tag table to select qualities that sound interesting for a world, or else roll 1d6 and 1d10 to select them randomly. (SWN, p. 95)
If you roll up two worlds both with airless atmosphere, burning temperature, microbial life, hundreds of thousands of inhabitants, 20th C tech, they suddenly become very different when you add the tags Tomb World & Trade Hub to one and Unbraked AI & Quarantined World to the other. Tags are also associated with other lists in the book that you can use to quickly build out the actors and forces on a world:
Each tag includes associated entries for Enemies, Friends, Complications, Things, and Places that fit with that tag. […] For example, two random tags for a world might be “Regional Hegemon” and “Psionics Fear”. Combining the “Colonial Official” and “Mental Purity Investigator” entries from the enemies lists of those tags, we come up with Heinrich Stalt, Imperial Mind Proctor, a grim inquisitor dispatched to the worlds under Imperial “protection” to purge them of the awful curse of psionically-active monsters. (p. 95)
Then when you start considering that those two worlds are connected by spacelanes to each other and other worlds and by the interests of vying interplanetary factions, all with their own set of tags (also all randomly-generated or hand-picked), then you start getting a feeling for how rich the setting of SWN becomes very quickly during the world-building process.
On top of the extensive world-building tools SWN gives the GM, it's also designed with different campaign scales in mind, so it works equally well for a group who wants to star-hop and see the universe as it does for a group that intends to play out the entire campaign on a single world. Playing on a single world allows you as GM to design a campaign around the tech level and societal forces that interest you: if cyberpunk is specifically appealing, SWN's rules already support that genre – you just have to pull together the already-provided puzzle pieces to make a planet ruled by corporations, criminal syndicates, and tech-savvy hackers. The espionage supplement Darkness Visible expands the GM toolbox for creating interstellar/planetary underground factions and secret agencies that would complement a cyberpunk campaign.
Best of all, core Stars Without Number is available in a no-art free edition in PDF, so there's no risk in taking a look.
Though there are plenty of reasons to look at other systems, I think that being in-print, easily available, and built specifically on the philosophy that D&D mechanics are best for a new genre because they're so familiar and time-tested, is a combination from the field of available RPGs that is uniquely suited to your needs. Take a minute to check out the free PDF and see if you agree.