If the gate is not fit for purpose and was sold to you as a gate you need to modify to fit your Panel Van (and it's not fit for purpose) it can be returned.
If it was sold to you as a Ute tailgate and you were told you cannot modify it in any way (or no returns) that is a different story.
Consumer rights really isn't a grey area. But is important you prove your case. 9/10 the consumer wins with a realistic case.