"Secured enough" is obviously subjective and depends on your scenario.
Google do have an oAuth2 API which apparently provides secure access to protected Google Sheets. The challenges will be:
1) getting Power BI to work with Google's oAuth2 API - this might be possible but at the least it will be quite difficult. There are no obvious doco or blog posts to follow.
2) getting Power BI to read the result - it doesnt appear (from my quick scan of Google's oAuth2 API doco) that it provides the same easy method to return an Excel file as the result.
I'd be really happy to be proved wrong, if someone has the time and persistence. Right now I have neither :-)