If all else fails and the Victoria services are too busy to board, you could consider the alternatives of taking a coach to Heathrow and then the train/Tube in from there, or taking the GWR North Downs service to Reading and then GWR/Elizabeth line from there to London. The latter would require separate tickets split at Reading, since this isn't a valid route on a Gatwick-London ticket (and it goes outside the contactless/Oyster PAYG area).