Gelding can happen to any mammal other than monotremes (echidna, platypus) and cetaceans (sperm whale, orca, narwhal). The only "civilized" race that cannot be gelded are kobolds because they aren't mammals.
I once managed to geld someone with a bite in adventure mode
I think kobolds are actually mammals. They have no scales, can sweat, and have long external ears, which are all usually mammalian traits, and have hair tissue and materials and a "hair" mannerism token, even though the hair isnt actually called upon for any tissue layers or shapes/colors, so I imagine DF kobolds must have very fine, sparse full-body fuzz but no thick hair on the scalp or face like the other humanoids, or maybe only nasal/ear hairs.
Kobolds might be related to monotremes. The game doesnt mention breasts or lactation at all for civilized races, only a few domestic animals have milk, so theres no telling if kobold women have nipples/teats like placental mammals and marsupials or sweat milk from pores like monotremes. Im going with platypus lactation though, if they are a monotreme.
Echidnas, the other extant monotremes besides platypi, have a "pouch" though. apparently not a permanent pouch like marsupials, but they can carry an egg around on their belly by contacting their abdominal muscles. Both males and females. Amazing abs apparently. Could kobolds have a pouch?