Yes you should or possibly vouchers to use at a later date, however if the line opens then I suspect you won't.
It should depend on whether the train you booked the ticket on actually runs.
Executive decision already made, I am WFH tomorrow.
Ticket is good enough as a receipt so just check the website but if you have time I would try and do it in person, surely you are back again in the the not too distant future and you may well have to do this within a month of purchase as well.
If you purchased them at Gatwick then you need to get a refund from Southern Railway who sold you the tickets, assuming you used either the Ticket Office there or a TVM ?