These are games only GOG offers sine they're all old and on Mac run via DOSBox. Turns out there are very few Mac-compatible games exclusive to GOG.

Echelon (1988, $1.50, 75% off) - a futuristic flying simulator where you roam an alien planet shooting aliens, finding artifacts, and rescuing people.

Elvira Horror Bundle (1990/91, $8, 20% off) - the only $5-$10 choice. Combination of first-person fantasy dungeon rpg with point and click puzzle solver. Never really read the description but it and the reviews make it seem interesting.

A Kiss For The Petals - Maidens of Michael (2018, $19.25, 45% off) - the only $10+ choice. Yuri visual novel with some nsfw material.