I don't think Baby Jogger are allowing stockists to display the prices for some reason (other than John Lewis and Boots ). Most websites have a box that you type in your email address and they send the price to your inbox. From what I have seen the John Lewis prices are more than other online retailers are selling them for so worth a look at a good few retailers for price comparisons and have a quick look for discount codes which help bring the price down further. Usually you can get offers on Boots and additional discount codes and with the bonus of advantage card points.
Have you managed to play with either/both these models in the flesh?
The main pros of the GT are the adjustable handle and the forever tyres giving a better ride. For me I would say this is worth the extra money as the adjustable handlebar is great for differing height pushers and if you are ever going to perhaps use a buggy board with it.
However, the main compromise with these extras is the weight! The GT is a lot heavier than the city mini and this may be a big issue for you? And of course the price but it is up to you to decide whether it is worth the extra money.
Sorry that's probably not been much help but good luck making your decision and I am sure someone will be along soon to give you better advice :)

Liswee is correct re the prices not being displayed.
This may help answer your other questions. Ignore they are singles and pretend they are doubles. The double is basically the single x 2! I think given your age gap, the GT may be worth your while getting because you will appreciate the benefits of the push one to two years down the line!