Almost every app in gsuite including google drive, keep, classroom etc. are on material guidelines including the forum but why not Gmail?
Is there a particular reason for this?
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I don't have any proof to back this up. However, Gmail has been around for 13 years. There are probably some people who have been using it for a very long time and don't want to learn a new layout.
In addition, Google has a Material Design email client at inbox.google.com.