That is the question! We havent found John on any censuses.
As avm said, the Gravesend family may be a red herring, but worth pursuing to see.
Censuses suggest there is also a Henry Thompson, a few years younger than George, also born Aylesford, maybe he is a brother to the George. Married to Roseanna/Susanna, a barge builder.
(Sorry, avm has already replied but I'll post anyway)