Gideon said to them, I will not rule over you, neither shall my son rule over you: Yahweh shall rule over you.
Whatever this ephod was, it became an object of worship and a spiritual stumbling block to the Israelites (Judges 8:27). Thus Gideon became the second official sponsor of idolatry in Israel, as far as we know, Aaron being the first. He was doing what was right in his own eyes (cf. Judges 17:6; Judges 21:25).