In some cases, like a pay-as-you-go phone, the provider adds a Proxy server which interferes with Gabriel’s local web server.
If your phone has a proxy service configured, you may notice a HTTP 502 (Bad gateway) error when running Gabriel on a mobile network connection. You will not see this error on a wifi connection.
To resolve this error, you can remove the proxy server from access point.
Go to Settings > Wireless & Networks > More… > Mobile networks > Access Point Names > Edit Access Point > Proxy. Make a note of the domain name entered in the proxy field then remove it. (your internet access will still work).
Note: You should restore the proxy field if internet access does not work properly after this change.