Does the Google Analytics connector bring in full data or sampled data?

We request all of the data. If the underlying data is itself sampled or incomplete, then of course we'd have no control over that -- but we will get everything that the Google API can give us.

"If the number of sessions in the property over the given date range exceeds 500k sessions (25M for Premium)1, Analytics will employ a sampling algorithm which uses a sample set proportional to the distribution of sessions by day for the selected date range."

https://support.google.com/analytics/answer/2637192

I don't know if this means per query in Power BI (my guess would be yes).

This means you need to break down your queries and keep duplicating...not ideal, but it works.

http://community.powerbi.com/t5/Integrations-with-Files-and/Set-date-range-limit-on-google-analytics-query/td-p/990