sixty to 90 minutes to clear Gatwick, get downstairs to the station (add 15 more if you need to buy the ticket and even more if you need to ride the people mover from North Terminal to South Terminal where the station is).
Up to 15 min waiting for the next Thameslink train.
75 to 90 minutes for the train, connections, and DLR transport with no luggage.
So now we are looking at 2:30 to 3:30 to get to check in at London City, if your flight to Gatwick is on time and there are no delays on the train (for example, no trains through central London this Sunday on Thameslink). That's 9:35 to 10:35 when you rock up. What time does your second flight require you to be at London City?