The pump model may not be designated exactly 20 GPM. In the Goulds line the 18GS10 is designated 18 GPM 1 HP but when you look at the table or the pump curve http://www.goulds.com/pdf/7310.pdf you see that it delivers 22 GPM at 50 psi with 40 ft of lift.
You want a pump that is reasonably matched to your demand. It will be less efficient if you operate it at much less than the designated rating. The curves indicate the recommended range of operation.
The submersible or centrifugal pumps are the most efficient of the pumps that you might choose. A shallow well jet pump is substantially less efficient.
If you look at the curves the pressure (head) scale is in feet. One psi = 2.31 ft so you would want at least 50 psi x 2.31 + 35 ft = 115 + 35 = 150 ft; and add a bit for pressure loss in the pipe and distribution system.