Can you take the tube from the airport to Victoria Station. Is this easy to do with 5 people and baggage or should we take a taxi?
The tube doesn't reach all the way to Gatwick. The most popular way to get from the airport to central London is via the Gatwick Express train. Since your traveling in a large group, you might qualify for a discounted group rate. Check out their web site at www.gatwickexpress.com
Definitely do NOT take a taxi. It will cost you a small fortune and it will be very slow since you will have a good chance of being caught in a traffic jam.
As previously mentioned, the special group fare on the Gatwick Express can save you some money.
If you arrive on a weekday and won't leave the airport until after 10:00 (anytime on weekends), you can save even more by booking Southern trains DaySave tickets online (at least seven days in advance). Up to four people for 20 GBP. The standard single fare is 8.90 GBP. Here is a link: http://tinyurl.com/34w7az. Southern trains run every 15 minutes and take only ten minutes longer than the Gatwick Express to reach Victoria.