You must use a day when you redeem the GAD voucher. The voucher is only good for admission on the date you redeem it, but you can upgrade to a MYW ticket. That ticket will expire 14 days from first use, but you can add the no-expiration option to it if you want to save the bulk of the tickets for another trip.
The no-expiration option ranges from $19 for a 2-day ticket to $222 for a 10-day ticket, so it depends what ticket you are buying if you will actually save any money by upgrading the GAD ticket.
If you are trying to save the ticket because you have booked a package, the tickets with your pacakge will not expire until you use them, so you could save them for another trip. If you have an AP, you can apply the GAD voucher toward an AP renewal.