Aluminium weighs about a third of steel for the same volume and doesn't rust. The trade-off is lower strength: a 50x50 aluminium tube is much weaker than a 50x50 steel tube of the same wall thickness, so aluminium gate frames typically use heavier sections to compensate.
For batten, slat, and louvre infill, aluminium extrusions are the dominant choice in modern Australian residential gates. They come in a huge range of profiles (flat, oval, deco, scalloped) from suppliers like Dimet, ProFab, and Knotwood. Powder coat finishes hold colour for 15+ years.
For structural posts and frames, aluminium is a cost-tradeoff against steel. Aluminium gates eliminate galvanising (saving cost and time) but pay back in heavier sections and higher base material cost. Most fab shops choose steel frame plus aluminium infill as the sweet spot.