Gatwick doesn’t forget. Their system sends a Penalty Charge Notice (PCN) to your registered address via the DVLA. That PCN will cost you £60 (or £30 if you pay within 14 days). It’s like a grumpy letter from your bank, but with more red ink.
You can appeal it if you have a good reason—like “I was in a coma” or “my number plate fell off.” But genuinely, just pay the £5. Your time is worth more than arguing with a parking robot.