cast iron
I expect that you will pay a high price for aluminum. In the DR there are a large number of craftsman who work in cast iron. My gate is fairly large, about 8 feet high and about 12-15 feet long. It is on rollers and runs on a single track of iron that is L shaped and set on its open side. It has worked well for years. I give it a shot of spray lubricant from time to time to keep the rollers moving smoothly. It opens with hardly any effort just so long as I spray it twice a year. If you look at others gates you will rarely see aluminum. Good luck!!!