The-Business: They could sort the games listed in the newsletter by popularity with other people who bought the same genre and/or release years and spread the delivery over several hours in which the early recipients which buy something indirectly decide what the later recipients will get.
You know so little about recommendations I don't even know where to start.
Genre is a ridiculously pointless feature. Even racing, a genre that one would expect to be well-defined, has genuine racing games, crime sims, the highly abstract Race the Sun and the surreal adventure Vangers.
Release year is ever so slightly better but only because it correlates with computer specs.
Raw tags are no better. I wouldn't play a crime sim like Carmageddon or GTA, but I love Antihero (a thieves' guild sim digital tabletop game), but I don't like Desktop Dungeons or Guild of Dungeoneering (also digital tabletop games) because they don't take the story seriously. I love Anodyne but not Undertale, Arcanum but not Baldur's Gate.
Ratings and purchase data are both bad in really weird ways. Ratings are very, very sparse. Purchases only go one way. Playtime through the client and achievements provide some feedback but it's not linear either. I can play an adventure game to completion by the clock and hate it, and I can win a singleplayer campaign on easy spending 1/50 of the time racked up by competitive players and love it.
Purchase data is additionally spoiled by availability (some games are GOG exclusives), release timing, sales, bundle deals and such. It's also very uneven, as some games sell stupendously huge numbers and others might struggle to sell a couple hundred. But recommending top-sellers isn't failproof: one, there's already a bestseller list/sort; two, discovery is one of the goals of recommendations; three, top-sellers are already well-known and more likely to be passed over out of distaste rather than obscurity; four, GOG has obligations toward publishers and their own business relation needs -- only a few games are runaway bestsellers and if they promote those to the exclusion of less popular ones, 90% of rightsholders (the indies, the noisy ones) will be dissatisfied with their games' visibility on the platform.
FWIW, the top 35 games by user rating are all RPGs. All-time bestsellers have somewhat more variety, but they include the blatantly mediocre Dragonshard and Demon Stone, longtime Soviet-style makeweights to D&D bundles.
Going by my discover queue and wishlist on Steam, it, with its vastly better data collection (because of their huge audience and a more invasive client, not through any failing of GOG), makes bad recs 87% of the time. Sure, they filter out the Mostly Negative trash -- so does GOG, by not selling it in the first place.
The TL;DR is that while spot-fixing recommendations is certainly possible (for starters, get rid of series content in "people who bought this also bought", and account for specs), making more sweeping improvements is hard, and even if they achieve an astounding success by data science metrics, your experience isn't going to noticeably change.