Make sure than your phone camera's lens is clear and that the scanner captures the entire QR code.
If the QR code still does not work, you can check whether or not you have a stable internet reception. You can also try restarting your GCash app. If these do not resolve the issue, try restarting your phone.
If the QR code still can't be scanned, it is possible that your phone camera is incompatible with the GCash app and it won't be possible to do a QR transaction.