When filling their inventory with things to eat it seems like they'll only eat from one stack at a time, up to 20 items. So items that stack that are made of more materials seem to yield better rewards.
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