Hello,
for information it takes me some time to understand that when setting up a gateway in Azure via Terraform the azurerm_subnet ressource should have exactly the name "GatewaySubnet".
Like in the example:
https://www.terraform.io/docs/providers/azurerm/r/virtual_network_gateway_connection.htmlthis naming quirk issue was reported below:
https://github.com/terraform-providers/terraform-provider-azurerm/issues/875