Your quote from the Oxford Companion about Lady Gowrie is correct, but misses some relevant detail. The Gowries were the vice-regal couple in South Australia, then New South Wales, finally in Canberra. Their period in office in NSW was 1935-1936. There was a convention, still current, that the Governor became Patron of the Rose Society of NSW and, in return, the Society named a rose after his wife. Olive Fitzhardinge had long been a member of the Society, so she provided the rose, thus called 'Lady Gowrie'.
Alister Clark had already used her married name (not her title) for a rose: 'Zara Hore-Ruthven'.
The same convention produced Riethmuller's 'Lady Woodward' in 1959.