Ram pumps, a very clever idea. We have one by me at Errdig Hall. Visitors just see what they call "the cup and saucer"

the stream flows into the saucer, then drops down to the ram pump in the centre where the energy of the flow of the water is then concentrated.

it can pump water up to over 100 feet to the manor house. every 10 gallons in can pump 1 gallon up.
invented, or rather re invented by Joseph Michel Montgolfier, of the hot air ballon fame..
These manor houses often had small water wheels too, to drive electricty turbines to run huge electric fires, as these manor houses would get very cold.
I do love Pyramid theories, and as more and more develop, we may even get to the truth one day.

though in this instance,
if one could get the water source up that high to feed the GP, then there would be little requirement to pump it into a pyramid, just to let it out again under high pressure further up the pyramid. its maybe possible to create clouds or a heavy mist from the outlets if the pressure could be amplified.
Plus the picture does not show the grand gallery ( or resonating chamber as I like to call it ) being full of water, which would effect the back pressure considerably on the valve and probably render it unoperable. there is no requirement for it being there in this theory.
The Ram pumps I have come across would run a pipe with a constant diameter, then drop the water into a tank in the roof, controlled by a cistern, so when the tank was full, the ram pump would stall.
fianlly it does not explain the presence of the granite box in the kings chamber.
But, as I say, I love a good theory, and I can never accept that it was a tomb.
No doubt the GP was desingned to withstand a massive amount of internal pressure, given that it has 8 sides, then had a layer of stone over the 8 sides, its probably unimaginable just how much it could cope with.
water, the key of life, made up of the 2 most explosive elements known to man, yet when combined in this formula, can cool rapidly and put out fires.
If we ever get to the bottom of the real human story, we may find an answer ?

Don't look back. That's not the direction you are travelling towards.