Welcome to Android Central! I don't know for sure how it is in the Fire OS, but with Android, you can usually long-press a photo to bring up the multi-select option. So in the Gallery, long press a photo until more options come up. Then tap additional photos (a check mark will appear over each one you tap). Then tap the Share button.
In the Gmail app, it's a similar process. Long-press an email, then tap additional emails to multiselect.