As I said earlier barriers also won't stop AP users getting on the wrong train. Very easy to do now from paddington the only thing stopping you with say a London - Swindon ticket is the on train staff.
I do keep harping on about technology, but if we had smarter tickets that contained the time/seat information for an advance ticket - you could have the gate show up the information to remind people of the train they're supposed to take. If it's too early (and I accept there's a bit more logic required from the gates), such that your train isn't there/hasn't yet been announced, it can reject or flag up 'seek assistance'.
Better yet, with the idea of a smartcard system that can confirm when you've boarded your train and taken your seat (and so anyone that doesn't take their seat and scan their card against a NFC reader on each seat back will make the seat available to others, on the traffic light system mentioned elsewhere) would also be able to warn you that you're on the wrong train.
In fact, if you had on board ticket validation with smartcards then you could operate like some railways do abroad. Not that I think we could all be trusted, but you could have a PAYG system that relies on honesty, backed up with HEFTY fines if caught. Sweden seems pretty open (you only scan/swipe a ticket to enter a station, not leave) but they have a fine of around £120 (current rates) if you're caught without a ticket or pass.
But, for the time being, the gates aren't really there to deal with people using advance tickets and boarding the wrong train. Given we don't expect any leniency for those who leave railcards at home, you could argue that getting the wrong train is 100% the fault of the passenger who would, or should, have had everything clearly pointed out to them at the point of sale. Mind you, given I nearly boarded the wrong train at York (as the previous EC service was running almost exactly 30 minutes late, so turned up when our booked train should have), I suppose I should accept reasons why people can make mistakes!