We use the built in WooCommerce email template system. To override the email you should copy these two files:
/wp-content/plugins/pw-woocommerce-gift-cards/templates/woocommerce/emails/customer-pw-gift-card.php
/wp-content/plugins/pw-woocommerce-gift-cards/templates/woocommerce/emails/plain/customer-pw-gift-card.php
To here (you may need to create the subfolders if they do not exist in your theme folder):
/wp-content/themes/<your_theme>/woocommerce/emails/customer-pw-gift-card.php
/wp-content/themes/<your_theme>/woocommerce/emails/plain/customer-pw-gift-card.php
Once the files are in your theme folder, open them up in a text editor and make any changes. You can change colors, wording, etc.
You can find out more about customization here: https://docs.woocommerce.com/document/configuring-woocommerce-settings/#section-21
Hope that helps, let me know if you have any questions!