UA does have a section on their wed site concerning travel documents.
https://www.united.com/web/en-US/con.../passport.aspx
And on that page there is another link for entry or transit requirements.
https://www.united.com/web/en-US/app...aspx?i=TIMATIC..
Fill in your information and obtain the results. Filling in just the require fields, it did not mention any time frame for transiting GRU as long as everything is on one ticket. But I have seen other posts on FT that indicate 8 hours is the supposed max time, yet others report remaining longer than that without a problem.
Good luck, whatever you decide to do and safe flight.