In order to register with GCodes, you need to redeem your balance using the payment option available of GCodes. After redeeming you will receive your code/claim URL through your email (remember to check your spam folder as well) that will direct you to the GCodes website, you will be prompted to redeem the code and will then be able to register the account. If you are using OpinionAPP you will also find the payment available by selecting the Rewards tab.
More information can be found in their FAQ: https://app.g.codes/faq
For assistance with registering, account issues, or redeeming your code you can contact GCodes directly here: https://app.g.codes/support
Hope this helps.