Good day @Spark,
A couple of things:
You have flashed the OS and installed the autopilot, but have you parametrized it? The steps are indicated in the second part of this comment, please follow them.
After that, if you connect to the brain via ssh and run
screen /dev/ttyS0 115200, your terminal should start to slowly fill with scrambled characters (the characters themselves aren't important, the fact you get to see them is). Is it like so?
Finally, what lights do you see on your GPS? To show that it's got a fix it'll start blinking a blue LED, if you don't get that you'll have to do something about your (physical) environment, probably change locations. It needs clear access to the sky, it's also possible that you get interferences from near radio towers, for example.