The times you've suggested would be really pushing your luck - no way I would be comfortable with that. Traffic between central London and Gatwick is purgatory (yes, even on a Saturday morning) - and delays might happen. Planning to arrive at LGW after a long coach journey only an hour before your flight is knife edge stuff - you have zero contingency time for delays.
Train is much faster. The trains from Victoria railway station (a short walk from the coach station) take about 35 / 40 minutes. If you have a contactless bank card, that's the simplest way to pay. Just touch in / out through the platform barriers using your card and you'll pay £8 for the journey to Gatwick.
Do not use the trains branded "express" which cost more than twice as much but only save 5 minutes.
Even better change your plans so that you get to Gatwick the night before.
Search on the web for customer reviews of easybus - no way I would dream of relying on them for a time critical journey.