According to the GEX website re single tickets: "You can use these tickets onboard anytime after the day you book, up to 30 days after the date you originally choose to travel. For instance, tickets booked on 1 May for travel on 15 May are valid from 2 May until 13 June."

So I can't see why you can't book your tickets now for a date of 30 June, say, and then use them anytime until 30 July.