You could use steel, my parents have a steel tube, built round with brick to match the front wall.
What sort of gate is it, do you know how heavy it is? Obviously a 6ft gate exerts a lot less force than the equivelent 12ft gate, only a third infact. But my gate is a light gate for a fully panneled afair, and a 6*6 is only just under half the strength of an 8*8. Where are you looking? I got mine from agricultural merchant.
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