There's no real need to travel first class unless ££ is not an issue (first class often does fill up anyway with people who don't pay). Getting on a train at Gatwick you might get a seat standard class - it can already be full but East Croydon is where it will get properly full. But it's not a cattle truck unlike some of the other routes. A lot of Southern Railways rush hour trains don't stop at Gatwick and you might also have to change at East Croydon as most SR trains go to Victoria. First Capital Connect may work better for you.