There's an impressive analysis on gamedev subreddit done by one dev about measuring and predicting game performance on steam. The key takeaway is that game awareness, measured by the number of organic followers on game-related steam community group, just before the release is the strongest predictor of sales. "Organic" means artificially inflated numbers won't do, and followers are organic if numbers are corroborated by discussion activity and twitter followers.

Gog minimalist equivalent to pre-release steam group is community wishlist. I wonder if gog could find ways to expand on such not-yet-on-gog game community pages and make them more visible to channel and measure organic interest. That would better signify to developers if they should expect good sales on gog and thus consider releasing their game here. Not many people see or use the wishlist and maybe many devs are unsure if it's worth their while to release and support on gog.

P.S. The analysis: reddit.com/r/gamedev/comments/buuire/what_factors_predict_the_success_of_a_steam_game