So,....Is the pressure gradient over the upper surface of the wing the cause of the seperation point moving forward with an increase in AoA?
Yes
Its called an adverse pressure gradient in the trade because the air flowing rearwards over the top surface is meeting air at an ever increasing pressure (ie the stuff that has curled round the trailing edge from underneath and starts moving forwards from the trailing edge) which causes lower pressure flow to separate so that it can take the easy route over the top.
Bye