Gachas are solitary creatures (Aside from having only ONE mate near them). If you have ally looking enabled, they will look at you if they're happy and look away if they're sad. Place them 12 foundations apart to make them produce and feed them owl pellets to up the rarity/amount of item.
Gachas are essentially living lootboxes and resource farms. They will eat anything (including stone, wood, or fiber) and turb it into crystals which can drop a variety of resources including black Pearl's, organic polymer, or obsidian, among other things. They also have a rare chance to drop higher-tier crystals which turn into upgraded armor and tools