Gift Certificate purchases are not subject to sales tax. That's standard for the purchase of any type of gift card, not just ones sold through MassageBook.
When a Gift Certificate is redeemed, the amount of tax due (if applicable) is calculated at that time based on the cost of the Service or Product that's being purchased.
Note: Service Certificates are good for the dollar amount of the Service's listed price in your Service Menu. If a Client is redeeming a Service Certificate and your Business adds tax to the Service's price at checkout, the gift code will only cover the cost of Service, not any additional tax.
As an alternative, deactivate the Service Certificate, and create it as a Custom Certificate that's valued at the cost of the Service + tax. The purchaser will pay that price, and the gift code will be good for it as well. When redeemed, the gift code will cover the cost of the Service + the additional tax that's added at checkout.