I have a few different methods of finding gates for different airliners. I like to replicate real world routes at times, and try to match the type of plane, the company, and the route. For this, websites like Flight Aware are great. You can just type in either a route or an airport, and go through the lists of departures to look for your plane model. I just had a look at Heathrow, and the first A350 I saw was a British Airways plane, Flight number BAW93, flying to CYYZ, Toronto. A lot of the time Flight Aware will tell you the gate number at the departure airport and/or the arrival airport. Also, you can click on track log to see the altitude that flight flew up to, along with coordinates of the route.
Another way if you're not too worried about exact gate numbers to match what each company uses is to check online for an airport map that shows which terminal is the international terminal, and this will generally be the terminal that uses larger aircraft, like the A350. Some Navigraph charts may show this, but usually they are just identified by a letter or number. A third way I have used in the past is to take a look on Google Earth or Maps, zoom into the image, and look at which terminals have which type of aircraft at them. You'll be able to spot the international terminals pretty easy like this, and gates which suit larger aircraft, and then you just compare where that terminal is to your Navigraph chart to see what the gate numbers are or just the identifier for the terminal. Sometimes you may be able to see the gate numbers on the satellite image.