The message
from "david lang" contains these words:

Sponix suggested; I'd put a post in at 45 degrees, running flush with the
fence, which
attaches to the upright about a 3rd of the way up.

If its flush with the fence how would it interfere with the car park space?

ISTM that the fence itself should already provide some support to the
gate post. Unfortunately when a gate is opened such support does very
little to stop the gate rotating(1) the gate post until the far end of
the gate drags on the ground.

(1) Or in the unlikely event of the footing actually being secure
bending the gate post, which might be the problem in this case.

--
Roger Chapman