Dargo,
There are vinyl gates, depending on the style you want, that already contain aluminum inserts.
Aluminum post inserts are also available for gate posts. They look like an H, this allows inserts into the post.
Eddie Walker,
Nice looking job. I am currently doing the exact same job on my farm. I wanted a maintenance free, or as close to maintenance free as I could get, fence and gate system. After a lengthy investigation into vinyl gates I fabricated my own. I used 2"*5" aluminum tubing. I built them with the same dimensions as the 3 rail fence. I used 5"*5" aluminum box tubing for posts. I had everything powder coated to match the fence. When you look out into the pasture you can not tell where a gate is, it looks like continuous fence. The gates are super strong and are as close to maintenance free as I could build. For hardware, after an even longer investigation, I made the gates cantilevered rolling. I used a track system for commercial gates. It consists of trolleys mounted to the posts that the track rides in. The rolling gate solved a huge amount of issues. The gate rolls back onto the fence line so there are no space issues. Rolling terrain did not matter. Worrying about the swing of the gate was eliminated. We all know the weakest part of the gate system is the post, this eliminated that problem. For fasteners I used stainless. For the latch I used a Sure Latch, made from stainless. As you can tell no, or very low, maintenance was the goal, I think Iam almost there.