Chalk and cheese
The 1966 Thunderbirds annual gives the ages of both John and Gordon as 22. This gave me the intriguing notion; what if they were twins? Not strictly canon, I know, but an idea to play with, and one I might revisit at another date - if my muse permits.
Two heads, one blond, one copper, bent over the chess board.
When Lucille told me that her third pregnancy was to be twins, my first thought was two lookalikes; how wrong I was! These two couldn't be further apart; one slim, one muscular, one quiet and intellectual, the other noisy and extrovert. One looks to the outer reaches of space, the other explores the ocean depths.
Though when the house is in uproar following one of Gordon's practical jokes, I sometimes wish they could both have been like John
But there again, I could have ended up with two Gordons.