I have sat in a waiting area, ordered from my laptops tickets that weren't available via the front panel of a TVM▸ , and collected them within a few minutes.
I have flown into Gatwick on several occasions and used the
FGW▸ app to buy and collect tickets. I tend not to buy in advance as I don't know when I'll clear immigration. Once through, I then decide which of the two possible routes I will take and then buy on the app. A short walk to the station, ignore the extremely long queue for the ticket machines and go straight to the ticket collection machines where there will be no one (usually) in front of me. Then down the escalator and on to the train.
Additionally as Grahame has done I will also buy journeys on the app that aren't available on the machine.