I’m assuming you’ve found your way here because you too are having difficulty viewing image attachments in the Gmail app for Android?
I found that thumbnail previews of images were displaying fine, but when I tapped to view the full image the progress bar would scroll around indefinitely.
In my case the problem was caused by Android’s Download Manager, and the solution was relatively simple.
Go into your Android Settings screen, select Apps, swipe to the ALL tab and scroll down to Download Manager.
Tap on Download Manager and then Clear data.
Exit the settings, go back into Gmail and hopefully you should now find that attachments are working again.