OH&S = Occupational Health & Safety, not Owners Health & Safety.
The OH&S Act was replaced by the WH&S Act from 1 January 2012.
WH&S = Work Health & Safety.
Neither Act applied/applies to purely residential strata schemes.
That is not to say that OH&S principles are not good guidelines for residential strata schemes.
OverIt, you [haven’t given] enough information to enable a considered answer.
How about telling us:
1) Was this same gate there when you bought the property?
2) Did your disability arise before or after you bought the property?
3) Is there anything defective about the gate other than that a person with a mobility issue finds it heavy and awkward?
4) What does “swing arm 900mm shy of the width of a single drive, steel reinforced frame” mean?
5) What reason did the OC (or other Owners) give for refusing your offer to pay to have the gate changed?