That would depend on what ticket(s) you've actually been sold. There's at least one split ticket in there from the price you've been quoted whilst the £27.50 is the price of an Off-Peak Day Return (so valid on any train 0930 onwards Monday - Friday, anytime on bank holidays or weekends). If you can say what tickets you've actually got we can advise on whether you're bound to specific trains or not.
Looks like you've been sold, from what I can see by doing my own search on Trainsplit's journey planner, a Purley - Gatwick Airport Off-Peak Day Return and a Gatwick Airport - Lewes Off-Peak Day Return. Those would be valid, on a Bank Holiday or weekend, on any trains as long as it calls at Gatwick Airport. But it really would be good to know for sure what tickets you've actually got.