The main difference between a Gift Subscriptions and a Prepaid subscription is that the gift has a voucher. Though both plans are for a predetermined amount of time, the gift plan starts whenever the voucher is redeemed. When a recipient redeems their voucher, they set up an account for themselves with their email and address.
Another difference is that prepaid plans renew automatically at the end of the term once they are completed. Gift subscriptions do not renew.