Hi Roger,
I made a gate just like that but 3m wide about 22 years ago. It still
functions fine. I used 50x25x2.5mm gal RHS for the frame. The frame was
painted as well.
20x20mm is way too small IMO, for the weigth of this gate.

You also need a good hinge system, I used a 8mm pin that fits top and
bottom into the horizontal frame, close to the edge. This really should
have had a reinforcing tube or a block inside as the pin hole wears
away over time. Otherwise, this type of hinge is perhaps the strongest
simple method of hanging the gate.

Weld the rectangular frame first and then mark and cut the diagonal
brace. If you cut it first it might not fit as I found the hard way.
Things move a little when welding them together.

Klaus

Roger, re the hinge system I mentioned. You will find that most fridge
doors are hinged similarily.

Regarding the diagonal brace, I read the responses with interest. My
gate has the brace in compression, I carried that idea over from a
wooden gate I also made. The steel gate frame never sagged yet with the
diagonal in compression. This gate also survived my 2.3 ton 4WD rolling
into it (hand brake failure), knocking the gate off its hinges. The
frame held its shape, the 50x25RHS is on edge ( narrow side up) BTW.
The thickness was chosen for ease of welding, it is VERY easy to blow
an unsightly hole into thin wall RHS with a stick welder and a mediocre
operator :-0. The colourbond steel cladding was pop rivetted on, some
of the rivets have been replaced in the 22 years since this gate was
first hung.
Klaus