A gift voucher must be used in accordance with its terms. Check your gift voucher's validity period, which shops accept it and whether it can be used to buy discount products. Also find out whether the gift voucher must be used at one time, if it can be split over several transactions or if a cash refund can be claimed for the unused part of the voucher's value.
If there is no clear mention of them, you can assume that there are no specific restrictions regarding the validity period, for example. It is also general practice that the consumer can order goods or reserve a service during the validity period of the gift voucher, since it may be difficult for the consumer to understand the seller's reservation status and when the reservation should be made.