I've been growing Gherkins outdoors for about four years or so now.
I save my seed each year, but they are based on the French ones called Cornichon de Paris.
Plant about six plants outside, three plants in a rectangular 12 inch pot, and train them up canes to a few feet high.
I get about twenty gherkins from each plant. I pick them when they are about 1-2 inches long. But leave them and they can grow to 4 inches, but then lose the gherkin tang, so more like a cucumber.
I eat them as they are, but also pickle a load. A staple through the when pickled for my homemade tartare sauce.