Russ,
Here's a description of a gate mark followed by a picture and a link to the website where I found the information. Hope it helps.
"This next generation of cast iron, beginning around the mid 1700's, have a "gate" mark, which looks like a long thin line on the bottom of the piece. This is where the iron entered the mold, and the technique lasted from the mid-1700's to the late 1800's or so. Pieces made around 1875 to the present time were engineered so the iron entered the mold from the sides. It entered usually in two places, sometimes opposite one another. These pieces have a smooth bottom. Current techniques have the iron coming in from the side, so look for one or more places on the sides of the iron pot with heavy grinding marks where the iron entered the mold. The grinding is done to finish the piece so that no unsightly burs or roughness mars the surface. This is a fairly modern development brought about by the ability to cheaply grind ironware. This would have wasted too much valuable time and been next to impossible for an 18th or early 19th Century founder to carry out."
http://www.ramshornstudio.com/iron_218d.htm