How do I paint a metal sheet fence with a spray gun?
You'll have some waste with spraying. Considerably more than with a brush or roller, anyhow. Find the square footage---both sides---and divide 325 into it. You'll get about that amount, 325 square feet per gallon, if you're careful. You will likely need more than one coat…

Also, make sure the surface has been well prepared by removing loose paint and sanding rust spots. You'll want a good primer, preferably oil-based, on those bare spots.