The easiest thing to do is when you get home, let him jump in the van with you to drive in through the gate. I used to do that, then it really becomes about you being home and getting to go for rides in cars, not about the gate being open.
He doesn't like riding in the car all of a sudden...won't just jump in.
I did that at first, but apparently now he's decided cars are bad.
Grrrrr. I know part of it is just that he likes playing over there, and now he's figured out the side gate isn't the only way to get there. But...grrrrr.