This term is used by the people of the Six Nations Territory to refer to something that is stupid, or to insult somebody. When used as an insult, this term refers to somebody who acts white, acts caucasian, or acts like they are not from the Six Nations Territory (people of the Six Nations Territory have a beautiful brown native complexion).
It is theorized that this term stems from a Cayuga word (one of the Six Nations languages) which means white. The word is pronounced like "gantgant", but is written using symbols not found on ordinary fonts.
Holy heck she acts gant
I'm not his friend because he's always acting gant