Gachas are happiest when alone or with a partner of the opposite sex. 2 gachas of the same sex close together will make both gachas depressed and they will no longer drop crystals.
Gachas will eat almost anything, including filled cryopods. Feces may be the only thing they will not eat, although they will collect any feces within their reach. Putting a gacha on roam in an area with other tames will allow the gacha to collect the feces for you.
Snow owl pellets are a favorite of the gacha, and their back crystals will sparkle after they have eaten one. If they have several in their inventory, they will not eat another pellet while the buff is on-going.
Gachas drop crystals that will contain 1 of 6 resources (each Gacha can make 3 common resources, 2 uncommon, and 1 rare or very rare resource) or occasionally a higher level crystal that may contain other items. Here is a complete list of everything the gacha can drop: https://ark.gamepedia.com/Gacha_Crystal_(Extinction).
Gachas will gather all the resources within their reach, swiping trees, rocks, plants, etc. They can also carry metal, stone, wood, and fungal wood at 50% weight reduction.
Baby gachas will eat from an adult gacha, making it easier to regulate their food (even the babies have a huge appetite).