Any suggestions how to get from the city (near the tower of london) to the Gatwick airport?
Take either the Circle line or the District line from Tower Hill to Monument. Transfer to the Northern line and take it to London Bridge. Take a First Capital Connect train from London Bridge to Gatwick.
Southern trains run from London Bridge station to Gatwick, but takes about 45 minutes with one change. Otherwise, the fastest way is to take the Gatwick Express, which leaves from Victoria Station, one tube ride away from Tower Hill. First Capital Connect trains also leave from Victoria and take around 35 minutes.
Assuming your talking about the London Bridge rail station, then there are direct trains from that station to Gatwick (travel time about 30 minutes). Be advised there is also a tiny airport in London called the "London City Airport" which makes your post a bit confusing?
I need to correct one thing in Jennifer's post. As I said in mine, First Capital Connect trains go from London Bridge to Gatwick. FCC trains do NOT go to Victoria. FCC trains make the trip in 30 minutes from London Bridge and are much cheaper than the Gatwick Express from Victoria. Southern trains are also cheaper.