They put him under guard, because it had not been explained what should be done to him.
Numbers 15:34
Surrounding context
15:33 And those who found him gathering sticks brought him to Moses and Aaron, and to all the congregation.
15:34 They put him under guard, because it had not been explained what should be done to him.
15:35 Then the LORD said to Moses, "The man must surely be put to death; all the congregation shall stone him with stones outside the camp."