Okay, let's start with the most obvious theory...
#1. Glenn can slip under the dumpster.
#2. Even though they're out of bullets, Glenn still has the flare gun. He can wait until after the zombies have gorged themselves then set off a flare, making them shuffle away to the bright light. And voila, back home in Maggie's arms in five minutes.
#3. What's the fun of being stuck in a NEARLY unescapable death without a surprise rescuer? But who has the skills and the lucky coincidence to be not too far from Alexandria? Bingo! Enid could either be inside the building behind the locked door Glenn couldn't open, or in the woods on the other side of the fence. Just survive somehow. That's right, girl.
#4. It's all a dream. Having a gun fired that close to your head and then falling to the pavement with a dead guy on top of you, Glenn obviously hit his head and it now dreaming. All right, that's a bit far fetched, but it could be possible...
Do you think Glenn can escape?