If you go into Settings > Safari > Advanced > Website Data and delete the Gmail.com or mail.google.com entries, it should delete any caches you ...GMail in Safari stuck in HTML mode - Apple Support CommunitiesUsing Gmail on Safari prevent shutdown wh… - Apple CommunityAutres résultats sur discussions.apple.com
Try resetting Safari. Close Safari completely from the app switcher window by double clicking the Home button and swiping up the Safari preview pane until it disappears from the screen. Then go to Settings > Safari > Clear History and Website Data. Then restart the device.