Gallons per minute to square feet per second?

Considering the gallon is a unit of volume (3-D) and square feet it a unit of surface area (2-D), the two are not comparable in this way. Gallons per minute is a rate of flow. Square feet per second is a rate of speed.

You may be referring to gpm /square feet - in other Words, the amount of liquid being applied to a specific unit of surface area.

Example:

You are flowing 1000 gpm and you want to know the density of that liquid given an area, say a 100' tank. The surface area of the tank = pi * (50)^2 = 7853 square feet. The density of that liquid = rate of flow / surface area = 1000 gpm/ 7853 sq.ft. = 0.12734 gpm/sq.ft.