Gatwick Airport

It's not south even a little bit, it's straight east in the docklands area.

You can get there many ways, overground rail to Shadwell may be the cheapest. Personally, I'd find another hotel. It's 3 trains + light rail, 3 transfers to get there, but if you tried hard you might be able to do it in 2 transfers.

Thameslink (train) to East Croydon Station
Southern (train) to New Cross Gate Station
London Overground (train) to Shadwell Rail Station
Walk to Shadwell LR Station
DLR (light rail) to Royal Victoria DLR Station (don't shorten "Royal Victoria" to "Victoria"-- entirely different in every respect!)

Departs 13:03
Arrives 14:22
Total time: 1hr 19mins

I looked at Google Maps and the hotel seems to be 'way east of most London attractions, while Gatwick is 'way south. The search function on Google Maps for Directions will offer alternatives; click on the image of a train to get the rapid-transit route. It will likely include a commuter train with a change to the Underground. "Rapid" still leaves you spending a lot of time on transit, and that applies to visiting the usual places in the centre of the city. Be sure to enter the time of day you want to travel.

I assume you have a convention at the Excel center or need to be close to the London City Airport. If you are visiting to see the big sights in London, this hotel is way off the beaten track.

Oh my goodness! Thank you for your advise.

Yes, unless you're attending some event at Excel, that's a very isolated and rather uninteresting area. I would see if you can move your booking to a different hotel.