Thank you all for your submissions. It's a tricky one I know; that's why I asked such a helpful and knowledgeable list!
No middles names that I have confirmed but Margaret's middle name may have been Jessie (or maybe a nickname?). As I said, her maiden name may have been NICHOL, as later on there was a granddaughter with Nichol as a middle name. The daughter born on the voyage was Elizabeth (born April 1837).
The shipping records say John and Margaret sailed from Dundee on 25 March 1837 and arrived in Port Jackson (NSW) on 13 July 1837. John's occupation was a Mason.
On the Friends of Dundee City Archives - 1831-32 Dundee Sassines records
http://www.fdca.org.uk/FDCABurghRecords1.html lists:
Gellately John; Mason, Hawkhill, Dundee. Wallace Craigie; south side Crescent Lane; Sassine No 350.
What is/are the Sassines

I also found the following on the Friends of Dundee City Archives but it could pure speculation

Howff Graveyard of Dundee:
Gellatly Alexander, burial 19 Jan 1837; age 1 yr. 4 mths.; born Dundee; Small Pox; Son of John Gelatly, Mason.
Margaret died in March 1880 aged 69; cert says married at age 23;
This gives a marriage around 1834 which fits with the possible birth of the above Alexander. Cert also says there were 2 deceased males and as I only had one, this accounts for the 2nd male!?
Merry Christmas everyone

Libby