You may use Safari and Chrome remote debugging to inspect and debug your webviews in your app, just like you would in a regular browser.
For iOS please check https://appletoolbox.com/use-web-inspector-debug-mobile-safari/ - it is the same process for Webviews running within your app. Your app must be running on iOS Simulator locally, or on a device plugged into your computer.
For Android please check https://developers.google.com/web/tools/chrome-devtools/remote-debugging/webviews
For this to work on Android, you will need a "debug" build of your app. By default, GoNative generates a "release" build for you, so that you can pubilsh it directly to Google Play with no changes required. To build a "debug" version, please see Building Android from source.