You are talking about the decrease in Gravity with furthur distance from the centre of the Earth? Negligable effect
G-load is related to the amount of acceleration an aircraft undergoes. The
amount of G you can pull is limited at altitude, but the G for any given situation is the same.
The climb to pitch ratio is roughly proportional to the Mack number. At M 0.5, one degree in pitch give around 500 fpm. At M0.83 one degree gives around 800fpm. This is why it is difficult to fly accurately at altitude, and some companies specify autoflight abouve a certain altitude.
In the situation you describe, it would take only half a degree or so of pitch to induce the climb, perhaps you applied that slight movement without noticing it?