Could delete the VNG after remove all vNet Peerings with another vNet from disabled subscription.
Cannot delete azure virtual network gateway
I followed all the advice given on this link, which asks the same question:
https://learn.microsoft.com/en-us/answers/questions/1066426/index.html
The Gateway to be deleted was in a Failed provisioning state and could not be deleted using the Portal (including attempting to remove the owning Resource Group)
In summary I used the Resource Explorer and tried to reset the VPN Gateway state (see the attached ResettingKWFWMVPN.txt file for details). The process ran for about 40 minutes before returning the result: 'Failed'.
I then selected the VPN Gateway, opened the Actions (Post/Delete) tab, and clicked the Delete button. The process ran for about 40 minutes before returning the result: Failed
Finally, selected the parent resource group and attempted to delete that in a similar way: Failed.
We are still in the Trial period of the subscription so do not have a Support Plan