We were free to explore the gardens after our tour.
Worth mention, maybe, that as far as I know the gardens can only be accessed with a tour, and I'm not seeing anything on the Roman Guy/Tour Guy website that includes the Museums/Sistine + the gardens.
Fran, I wish I had an option to offer up but I'll caution you is to use the Vatican's own official site for ticketing/tours or other respected operators recommended by posters on this site (the Roman Guy/Tour Guy is one of those.) There are several 3rd-party sites out there that are cleverly designed to look 'official' but are overpriced imposters.
This is the Vatican Museums official website; they do have a garden and museum tour although we've not taken it.
https://www.museivaticani.va/content/museivaticani/en.html