I understood that g-lock was caused by prolonged exposure to high g, normally happening in a military jet.
Without training (straining techniques, breathing) or equipment you can get G-LOC at as little as 5G. You don't need a military jet for that. Any WWII military fighter, and any half-decent aerobatic trainer, will do 5G with no problem.
The G-LOC onset curve is complex but in certain situations it can happen in less than four seconds. You don't need "prolonged" exposure. See the figure in this article:
G-LOC, COULD IT HAPPEN TO YOU?