Albert Barnes Commentary Judges 8:27

"And Gideon made an ephod thereof, and put it in his city, even in Ophrah: and all Israel played the harlot after it there; and it became a snare unto Gideon, and to his house." — Judges 8:27 (ASV)

The ephod was that particular part of the high priest’s dress which was necessary to be worn when he inquired of God by Urim and Thummim.

It seems that Gideon, now being the civil ruler, desired to have an ephod of his own, kept in his own city, to be worn by the priest whenever Gideon might summon him to inquire of the Lord for him. His relations with the tribe of Ephraim probably made him unwilling to go to Shiloh. Compare the act of Jeroboam (1 Kings 12:28).