I believe I should take the Gatwick Express.
No, you need to take a "Thameslink" train direct to St Pancras.
Beware the ticket machines at Gatwick. If you just select a ticket to St Pancras, by default it will sell you a ticket via the Gatwick Express and Victoria, because this is ~5 minutes faster. But this is a lot more hassle as you would have to change onto the tube, up and down escalators, squeeze your bags onto a tube train etc., and is more expensive. The cheaper "Thameslink" route is direct with no changes.
Either get one of the attendants to help, there are usually 1 or 2 hovering near the ticket machines waiting to help, or go to the manned ticket desk and ask for a ticket to St Pancras via Thameslink.
How long will it take me to go through customs (flying in from USA) and catch the train?
Customs takes no time at all. What takes time is taxiing, de-planeing, walking through corridors (10-30 minutes), immigration (passport control (10-50 minutes), waiting for luggage (not long as you have waited so long in immigration), and, if you land at the north terminal, you need to go to the south terminal to catch the train (10-15 minutes). Plus your flight could be 1 hour late landing if the wind is in the wrong direction.
It is usually not a good idea to buy a fixed time rail ticket after an intercontinental flight. Too many risks.