Can I use multiple Google (gmail) accounts on a single Stream TV Box (for other users, for example)?

Yes, you can add additional Google (gmail) accounts by pressing the TiVo button on the Stream TV remote, then pressing left to access the Menu, then select Settings, then Device Settings, then Accounts & Sign In, then Add Account. You can then toggle back and forth between Google Accounts in supported apps. For instance, in the Google Play Store, each user can select their icon at the top right and then choose "Switch accounts."