When you say transparent proxy, you mean that no proxy is configured on the clients and the proxy setting on the gateway is disabled, right?
The only way to see if HTTPS traffic is not handled by the gateway is by looking at the certificate that you get on the clients, which is then signed by the original site.
When you enable HTTPS decryption you need to install a certificate CA on the gateway that is trusted by the clients and enable HTTPS scanning on the gateway. Now the certificate will be issued by the gateway not by the original site.
Regards, Maarten