Gnat QFIs I flew with would require of me sustained high g turns, which, age 20 and reasonably fit, I was happy to furnish them with but I did notice that after three or so rapid turn reversals of max possible rate (full thrust, quickly to max g [7] on the buffet, max IAS [360 kts, I think], bank angle approx 140 degrees and going down fast) I could still hold a conversation with the instructor but, meanwhile, had quickly and completely blacked out.
After overbanking and entering the spiral descent, I recall that it was "Pull to +5G, squeeze to +6", whilst looking over your shoulder at the number of fingers your QFI was holding up....
Those max rate turning sorties were quite physically demanding - but great fun!
At least the Gnat had a proper anti-G system which used bleed air, rather than the bottled air of the Hunter. On some range sorties it was possible to run out of anti-G in the F6A / T7, but IIRC the FGA9 had an extra bottle. Running out of anti-G when you least expected it was very disconcerting! One of our course mates* remarked that he'd completely blacked out when it happened to him in the T7 - something of an unfortunate turn of phrase though, given that he was of Jamaican descent.
*Great chap - went on to higher things on the Jaguar.