30 x 1 is what seems to be used over here. Rust is not a problem and some of these gates are more than 50 years old. From the photo (30 x 1) you see that the steel is left in its natural state for a couple of years to get the rusty patina, then if you can be bothered, treat with RUSTOIL which is a mix of varnish and Jenolite probably with a bit of linseed oil for good measure. One coat is sufficient for 20 odd years as a bit more patina wouldn't matter.

Buying lengths of steel gives a lot of wastage unless you weld odd bits together. Older gates were riveted at each end of the strip to the framework of 35mm x3mm steel tube (older gates were solid and forge welded at the joints). with a few rivets every 4th joint. I was going to rose weld the ends and spot weld every 4th joint. The gate in the leading photo is rose welded.
Using galvanise strip would look out of character and not be so good for welding.
I am not trying to re-invent the wheel here, just following tradition to fit in with the countryside. I did think that if I could find a 'Hooper' that they may know of a source of strip (Hoopers made the hoops for the coopers who made barrels and kegs etc), but havn't found one yet but they MUST be local as we are in wine country – probably made in China these days !
BobH