I've seen this issue in a few emails I've received recently and not sure if anyone has posted a fix for it so thought I'd share mine.

Sometimes the Gmail app refuses to respect the 100% width set on a table and instead just fits to the content. Even is the table already has width="100%" and style="width:100%;" set on it.

I'm not really sure what's causing it and can't seem to reproduce it easily, I actually ended up scraping someone elses code to reproduce the issue.

Anyway to fix this simply add style="min-width:100%;" to the offending table and it'll display as expected.