This story leaves things so open ended!
I wonder what happens. You have to give George a 10% chance against the gray matter monster (if that). So this thing is splitting, then will split again, and again, etc. It has no tolerance for light. So if you can keep light around yourself, maybe you will be OK... but maybe if enough of these things exist, then eventually it would be just too much.
It was also said that if you touch this gray matter, you become a monster, unless that was just a scare tactic by Richie. They did say the slime was all over the apartment house and there was no movement from the other floors of the house, and there were no lights. Were these people "infected" also? If so, I wonder if George got his shots off, but perhaps was overwhelmed by the others. In this case, it would be like a Zombie story... but with the possibility of splitting which would lend completely no chance of survival.
Thinking about it, I give humanity a very small chance of surviving this