Yes, a new Client can purchase a Gift Certificate online even if you don't allow them to book online.
Online Booking Rules and Gift Certificates are independent of one another.
If your Business is connected with either Square or Stripe for credit card processing, all Service Certificates and Custom Certificates that are Active and Online can be purchased online by both existing and new Clients through your MassageBook Business Website or Business Listing.
Once a new client purchases a Gift Certificate, they are added to your client list and will be considered an existing client when attempting to book in the future.