‘And Gideon made an ephod of it, and put it in his city, even in Ophrah.'
We do not know for certain what an ephod (a metallic sacral robe) in this context was for. In Exodus 28:6-35 it was a garment worn by the priests, which contained the precious stones which represented the tribes of Israel. It may thus be that this was such a garment, made of the purple robes of the kings, ornamented heavily with the gold, to be kept as a memorial of Yahweh's glorious victory over their enemy. There is no evidence for suggesting that it was an image, although it may have been placed over a stone pillar. Nor are there any grounds for thinking that Gideon initially encouraged its veneration. (If it was an image why did the writer not call it that?)
‘And all Israel went a-whoring after it there, and it became a snare to Gideon and his house.'
This probably indicates that the people saw it in terms of the priestly ephod and began to consult it like an oracle. The priestly ephod was connected with the breastplate which contained within it the Urim and the Thummim for the purpose of consulting Yahweh. It may then be that the household of Gideon encouraged this. It would bring them great prestige. This might have led on to veneration and worship of it by some of the people. Thus what was in the first place intended to be a memorial to the glory of Yahweh would become a snare to him and his house, and a stumblingblock to the people.
If it happened while Gideon himself was still alive it may well have been seen as a way of discovering Yahweh's will. They did not go a-whoring after Baal until Gideon was dead (Judges 8:33). But this did not make it right, for it turned them away from the central sanctuary which was where Yahweh's will could truly be found. Only the priest at the central sanctuary could consult Urim and Thummim before Yahweh. We can compare how the brazen serpent, later called Nechushtan, made by Moses at God's command for a good purpose (Numbers 21:8-9) also became a snare to Israel (2 Kings 18:4). Any religious object is open to this danger which is why they are best avoided however ‘nice and helpful' they seem at first.
But some suggest that we should translate ‘to Gideon, even to his house' (see Judges 8:34), that is ‘Gideon' as signifying his house (as ‘Israel' signifies the children of Israel), suggesting that its main harm occurred after Gideon was dead. Then this could be seen as connected with Judges 8:33 and be referred to Baal worship. Note that there is no specific condemnation of Gideon for what he did, only indirect disapproval of the result. It is a warning to all how easy it is to lead others astray with what at first appears to be innocent.