You have to throw something they eat at them. Gachas will also eat structures though if you drop stone foundations or stone fences in front of them they will eat it. I have found that if you drop stacks of 50+ stone fences they will eat it and the taming bar will skyrocket. It obviously depends on how fast you are and the level. You have to make sure you are feeding it fast enough since the taming bar drops so fast. Best way is to have several stacks of stone fences or an easy to craft stone structure and just feed stacks of it.