Another issue, that might be relevant, is the insurance risk if an accident occurs. Most strata buildings include insurance cover for owners, and committee members doing voluntary work on the building and grounds. But the voluntary work must be specifically authorised by the Owners Corporation, in the minutes of a strata committee or general meeting. If not so authorised, then the building’s insurance is unlikely to cover you if you get injured doing the landscaping work (which is a high risk area). Secondly, this only work for “voluntary work” which must be unpaid. If you were charging the Owners Corporation for the work, then it’s not voluntary.
So you should tender for the work, just as the new gardener did. This will require you to have the appropriate insurance, and business details in order. Do you have the appropriate insurance and other requirements? If not, then the Owners Corporation did the right thing by engaging another gardener, who the strata manager would have vetted. As an alternative, why not join the strata committee at the next AGM (or ask to assist them), and then become the committee member supervising the work of the gardener? But not to actually touch the garden, unless as a volunteer for no payment, and with the task then minuted in the strata records.