I need to construct an outdoor shallow ramp approx 4m long and 1.5m wide for a 1:12 gradient. I'm considering metal sheeting for the walkway rather than decking but I don't know what thickness of aluminium or steel would be adequate or considered typical?
I'm not sure "thickness" really matters as much as the overall design of the ramp. U-Haul trucks come with 12' long aluminum ramps that one person can lift and they are made of pretty thin aluminum. They are rated at over 500lbs because of the overall design of the ramp.
Thickness really only matters when talking about one single unsupported span, and that is dependent on overall design. You could have a 24" unsupported span with 1/4" steel or a 6" span with 1/8" aluminum (all made up numbers).
A better approach would be to find an affordable metal that is suitable for the purpose of a walkway, then decide what design would allow you to use that based on its weight and flexibility. Once that is done, you can look at the cost of a more typical surface (wood decking), compare costs, and see if metal really makes sense.
There are online plate deflection calculators. Here is 4 meters x 1.5 meters. 1.5 meters unsupported is a very long distance for a plate. I made some assumptions on the weight and allowable deflection. For 0.1 inch deflection (center displacement), and 500 lbs load (about 2 large people), you need 1/2 steel. This will be prohibitively heavy. JPhi1618's intuition is good, you probably don't want to use a plain plate.