"What exactly is the definition of GMO? "
Good question. I won't answer that.
"Did we do it without GMO food? "
If we only consider GMO as targeted modification of a DNA sequence using methods other than selective breeding, no we didn't. That technology has been around for maybe 30 years. The world's pop hasn't gone up 30% since.
"Isn't a lot of today's food the result of GMO?"
In the West, yes. But despite the promises of lower water usage, more nutritious crops, GMO foods seems to be mostly adding Round Up resistance to crops to enable more mechanized agricultural practices (not necessarily cheaper for the end user).
This is not necessarily robust and it promotes monoculture. Furthermore, financial considerations might make it more economical to farm something non-edible, or not farm at all [0].
[0] Yes, theoretically the price of food would increase to make it profitable again. However, the equilibrium would be at higher prices (pricing out the poorest) and lower Q.S. (i.e. it's good that the poor aren't eating, cause there ain't enough to go around). Besides, it takes time for the system to reach equilibrium, at which point there's a nice famine.