The Gatwick Express is running now. Disrupting strikes were earlier this month. What could happen in the future should be confirmed closer to your travel date.
As Claudia mentions, getting into London from Heathrow is arguably more convenient - faster, and cheaper. Buy your Express ticket for the train from either airport early, and you can save. The difference in price and time might not be a huge thing for you, and might not offset the price difference between Air Canada and WestJet, but the Gatwick Express takes 30 minutes, not the 15 of the Heathrow Express, and it costs closer to £30, where the Heathrow Express costs less than £6 … if you book your train ticket online, at least 90 days prior to your arrival.
Back when British Airways first started serving Denver, the flight to London landed at Gatwick, our only option. That worked out back then, as the Gatwick Express was relatively cheap, and it went from the airport directly to the Victoria Station train station, with our hotel a very short walk from there. Where are you staying? The price for the Gatwick Express has gone up a lot since then, by the way.
More than 20 years later, BA now flys from Denver to Heathrow, so again, unless we used another airline, that decision’s been made for us. The Heathrow Express, which is a relatively newer trainline, now gets us to Paddington Station, more convenient to the more northerly parts of London where we’ve stayed the last several trips. No more Victoria Station neighborhood for us.