Update 7/14/2018: Gmail has fixed the issue. Ids and classes are no longer being stripped.
It looks like Gmail Classic is currently stripping ids and classes from elements within emails.
Gmail rolled out a new version of the UI in beta in April, however it looks like in the process of migrating users to the new UI a few days ago the “classic” Gmail suddenly reverted to a pre-2016 version before ids and classes were supported. This article highlights the changes in the 2016 update.
This means that your email might render differently in the old and new Gmail especially if you’re using embedded styles (CSS in the <style> blocks).
If you’ve stopped inlining your CSS, you may have to consider inling your CSS again – at least temporarily.
If you’re on the new Gmail and want to check your emails in the Classic version you can
click on the Gear icon on the top right and select “Go back to classic Gmail”.
I’ve notified someone I know in the Gmail team and I’ll report back if I hear anything.
h/t to Mark Robbins for doing some initial research on this.