I understand what your mother would feel, but I'm afraid that she might not feel that way after getting to the airport.
The only reasonable route by car is south on the A23 Brighton Road which is a very slow clogged up urban road with traffic lights and horrible traffic for the whole journey as fas as Croydon, where it has a short blast as two lanes in each direction, and then back down to one to get through the infamous traffic of Purley, and eventually becomes a dual carriageway just before crossing the M25 and turning into the M23 for the run south to the airport, then slowly into the actual airport, and your walk from where the minicab has to drop you. There are absolutely no guarantees of times, and you will have to leave plenty of padding.
By train, there is a Gatwick Express train as often as every 15 minutes, sometimes every 30, and it takes 30 minutes, taking you right into the airport terminal. If you are in the South Terminal it is just an escalator or lift up one flight and you are right inside the terminal.
Look out the window at the last view of London from the bridge as you cross it, and soon you are whizzing south and it isn't long before it is fields with sheep and cows and horses, and beautiful views of the rolling countryside.
If it is raining the day you leave you will be dry from the time you enter Victoria Station all the way until you land at home (the Gatwick station has covered platforms but the fact that it is a through station does mean that the ends are open). If your flight is from the North Terminal you have a very short walk to the people mover which is an automated short train which connects the two terminals. It is scenic, with windows all around and plays ads at you. More than anything else it is like a horizontal elevator. You bunch up near the series of opaque doors and when they open you step in. A couple of minutes later you step out and you are in the other terminal. Really easy, heated, air conditioned and extremely frequent - every couple of minutes.
Gatwick Express rains are very reliable, and much faster than the car. Big picture windows, very comfortable seats in standard class, heated and air conditioned. The train zips along at between 100 mph and 110 mph most of the way, they car can only do 70 if it is the motorway at the end of the trip after creeping along at no more than 30 and stopping at all the lights.