Because zero gravity is fly but that's just basic physics
Gravity is a force that causes things to drop down to the ground. Without any gravity aka “zero” gravity" things would just float/fly around. It’s basics physics because it is one of the beginning fundamental things you learn about in Physics class.
Also, this is a reference to the mixtape this song is on: “Zero Gravity”. He’s saying this mixtape is fly aka extremely good, but that just goes without saying because it’s him spitting.
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