Changing the voice for your Google Action is done outside of Voiceflow!
Head to your Google Console: https://console.actions.google.com/
After selecting a project, there's a menu on the left. Go to Invocation, as circled below.
You can also access this menu by clicking "Decide how your Action is invoked" under Quick Setup.
You can scroll down to Google Assistant Voice, then select which voice to use.
At the time of publishing this article, there is no way to use both male and female voices in the same Google Action. This is a Google setting, and if/when it is updated, we will add that feature to Voiceflow!
By default, the box to match a user's language setting is checked. You can read more about Language and Locale support for Google here.