£76 sounds more like a Heathrow price. When I run an Oxford address into the site you list I get £115. Still this might be worth it for the convenience.
However, note that by train it is only £65 even at peak time if you get a 'Gatwick Flyaway' ticket. This obscure special is sold as a return originating from Oxford, but it is perfectly legitimate to use the return portion before the outward (although you could not then use the outward later). Valid for travel not via London, changing at Reading. You would need to buy it online from GWR and collect it at Gatwick.
If your journey from Gatwick starts after 9:30 then simply buy off-peak singles with Groupsave and that would cost £51.15 as 3 go for the price of 2.