Pathfinder's Decanter of Endless Water is a sad shadow of its former D&D glory.
The description isn't internally consistent unless you assume "spray" means that the water is mixed with air (or some other substance), and water streams are not usefully measured by length. But let's make some assumptions and see what we can do:
The decanter spits out a stream "20 feet long." For simplicity, pretend that's straight up in the air, which will give us the most force here. That says the initial velocity is reduced to 0 after subjected to a 32ft/sec deceleration, so d=(1/2)at^2 = 20 = 16 t^2, t is about 1.1, so this decanter is sputtering out about 36 feet per second, or 11 meters/second.
It's throwing 20 gallons of water per round at that speed, which is 3.3 gallons/second. A gallon is 8.3 pounds, so that's 27ish pounds, or 12 kg/second.
Put together, that's 132kgm/s^2, aka 132 Newtons of force. 1 Newton is roughly 1/4 pound, so its roughly 33 pounds of force from the water.
There's a little more, but it probably doesn;t change anything. 33 feet/second in a 1 foot diameter is about 26 cubic feet of stuff per second. The water occupies less than 1/2 cubic foot (the inconsistency mentioned earlier), so only about 2% of what the decanter sprays is water. What else might make up that stream is completely undefined. If it's air, it adds about 10% more force. If it's something else, the answer is even less well defined.