A gift voucher is for an amount (50 GBP, say, spent across bookings until it runs out) or for a named service (one Skin fade, whatever it costs on the day). Under Your shop, Sell more, Gift vouchers, put amounts and services on sale, and if you like let customers choose any amount between a lowest and a highest. Set how long vouchers last: 6, 12 or 24 months, or never. What is left at the end lapses.
On your booking page, a "Buy a gift voucher" link appears once something is on sale and card payments are connected. The buyer picks a voucher, pays by card through your own Stripe account, and chooses Send it to me or Send it to them, with the person's name and email, a message, and if they like a date for it to arrive. The voucher email has the code and a link to the voucher's own page, which shows what is left, when it runs out and a Book now button, and prints on one page. You get an email for every voucher sold online.
At the desk, press Issue a voucher: Sold now (paid by cash, card or transfer), Bought before FullDiary (type the old voucher's code and what is left on it, so the customer can use what they have), or Complimentary. Email it to whoever it is for, now or on a date, or hand over a printed one.
Customers type the code on the booking page's last step, or arrive with it from their voucher's page. The voucher pays towards the booking, the deposit first, so a voucher that covers the deposit means nothing to pay by card. An amount voucher pays as much as it holds and keeps the rest for next time; a service voucher pays for its own service. In the diary, open a booking and type the code into the Gift voucher box to take one at the desk; then Paid cash or card takes only the rest. The booking, the confirmation emails and an invoice made from the booking all show what the voucher paid.
If a booking paid with a voucher is cancelled, the amount goes back on the voucher. A no-show keeps it spent, as a package keeps a no-show's session. Cancel a voucher from the list whenever you need to; refund any payment yourself in Stripe. A voucher is a payment, not a discount: selling one is not takings until it is spent on a booking.
Gift vouchers are part of Pro. Vouchers you have already sold stay good if you ever leave Pro.