ZipGrade does not accept gcash payments directly through the website. However, you can first fund your device's app store account using your gcash (or any other payment method accepted by your device's app store) so you can then purchase the subscription through the in-app purchase option (which is processed through the device's app store). If you are having trouble doing this, please contact your device's app store support or the support for gcash payments on your device. (For Android devices: https://support.google.com/googleplay/answer/2651410)
For more information on unlimited grading purchase methods:
https://support.zipgrade.com/hc/en-us/articles/10196863139597-How-to-Purchase-Unlimited-Grading