Hi Mark, One of my go-to ways of figuring out whether a contact has bounced is to create a report off of the Email Logs object and be sure to pull in a field called "event messages" -- theses are the messages that are returned to the Gainsight email domain when an email is rejected by a recipient (these are the same messages you will find on Administration/Email Config/Email Validator). If you pull in Account name, Batch Name (i.e. Outreach name), contact name, email, triggered date, *search for Bounce, pull in hard and soft bounce, and create a filter for "event message != null" and that should give you a list of bounce messages.
That is the only way that I know of to access a bounce list that is internal to Gainsight. I hope that helps.