Gatwick Express or taxi?

It wouldn't be much cheaper to take a pre-booked taxi but it would take you right to your condo rather than having to go from Victoria Station to the condo as well.

Don't take a taxi you hail at Gatwick as that will be the most expensive. Pre book. Probably around 80 GBP.

As Emma says, the journey from central parts of London to Gatwick is long and tedious - 75 minutes maybe at best at worst much longer (and one of the theoretically quickest route tantalisingly takes you past Heathrow!). Train is half an hour.

Don't bother with the so-called Gatwick Express; these days the other services are only 3 minutes slower and would cost a party of 4 adults £10 less in total.

Our family of 4 took the Gatwick express last summer. It was such a fast way of getting to our condo

Just took Uber from near Sloan Station (1 away from Victoria) to Gatwick last Friday (3-23-18). Black Cab was quoted at 95 pounds, Uber between 55 and 75 pounds (it depends on time and therefore traffic). We (my husband and I) left London at 6:15 a.m., it took about an hour and it cost 61 pounds. Gatwick Express was 20 pounds each plus we would've had to pay 2.40 pounds each for the tube as our 7 day pass had expired. Also we would've had to roll our suitcases at 6 in the morning to Sloan (10 minutes away), paid for another trip on the Oyster Card, at Victoria buy tickets for Gatwick, then find the right track and deal with wherever they let us off. At 45 pounds versus the 61 pounds of Uber it was well worth it to have them show up at the door, load our luggage and bring us to the right terminal at Gatwick. I'm still pleased with our decision!

Unfortunately the OP doesn't say what the expected time of departure is.

Also the fare to Gatwick from Sloane Square all in is £10.60 before 6:30am if paid by Oyster / contactless.

However, the extra cost at that time of day may well be worth it for the convenience, especially if not staying on top of the relevant station(s).

I'd go with Gatewick Express. Last November we were there, in an apartment just south of Victoria Station also. We had driven into London at the end of our trip with our rental car, dropped off our luggage then my husband returned the rental to Gatwick where we'd picked it up. It took forever for him to drive to Gatwick, over 2 hours later in the day on Sunday afternoon, which you would except to be faster with no computer traffic but it was slow. He came back on the Gatwick Express which is what we also took a few days later to get back to Gatwick to fly out. We just walked to Victoria Station and zipped right to Gatwick. No stress and fast. For an 10 am departure I'd get a hotel right at Gatwick for Saturday night and leave from there. Personal preference but that's what we do. Either way, enjoy your trip!