“Chemosh your god gives you.” Chemosh is normally viewed as a Moabite god, but it is conceivable that he was worshipped by Ammonites as well. It is highly unlikely that Jephthah recognized Chemosh as a legitimate god. Jephthah almost certainly said what he did about Chemosh to avoid an argument about the Ammonite god, and also to get agreement that if a god gave territory to you, then it was indeed your territory. If the king of Ammon agreed with that statement, then he would have to agree that the land he was invading belonged to Israel because it was given to them by Yahweh.
“your god.” The Hebrew text uses the plural, literally, “your gods,” but it is a grammatical plural, not that Chemosh was a plurality of gods.
“whatever.” The “whatever” includes the land and anyone on it.