Any untrained person exposed to 5G would probably 'grey out' as you describe. That's not the same as G-LOC which is a much nastier thing altogether and happens
after the high-G exposure, not during it.
I certainly 'greyed out' at the age of 15 on an exciting ride. In the coal-hole of a Sea Vixen at the Farnbrough Airshow during a simulated attack. I remember thinking that it was like looking through a net curtain at the world outside.
God knows what nanny-state rules that would break today!
Don't worry - and get as fit as you can before you join. They seem to like that these days....
