I wonder if he had some experience in the past where the stirrup caught on something when going through a gate and it really scared, even hurt him?
Have you tried backing him through the gate, so he can't see it?
Maybe you need to set up something that simulates that sort of postion, like a vertical post next to the round pen (inside) and work on moving him between two things that '"pinch" him, first naked then saddled.
Just got my OTTB to go in a round pen tonight. Don't force her lead him/her to it and walk the outside of the pen ,got to the gate try again, walk the outside of the pen touching it and talking to you horse, Then I had my wife walk in ahead of us an then Zena (my horse) followed. Im not a trainer or horse expert but we just bought our OTTB 3 weeks ago and she was left to pasture for a year and she was very spooky. Lots of patients is the key, Let him or her know its ok.