So if you try to preorder an item from an online store that uses Shopify as their web platform, you may get the message saying you can not use a Gift Card to pre-order with Shopify, when you click the Apply gift card button. This may happen if you are ordering from the stores online website.
However, to get around this, download the Shopify "Shop" app to your phone. Log into your Shopify account from the Shop app, and go to the Store you want to place an order from with your Gift Card. Add the pre-order product to your cart and checkout. Enter your Gift Card, during checkout and make sure it is using the Gift Card amount when placing the order.
Note: If you have an issue, and have never used the Shopify app on your phone, it may be necessary to add your Credit Card to the Shop app first, before they accept the Gift Card when checking out.
You may also get Exclusive discounts to the store you are purchasing from when ordering with the Shopify Shop app, so it's worth trying if you don't already have it on your phone.