Chrome doesn't run extensions by default in incognito mode.
> Because Google Chrome does not control how extensions handle your personal data, all extensions have been disabled for incognito windows. You can reenable them individually in the extensions manager.
Keeping your gmail tab in an incognito window might be a good approach.
While it's a decent workaround, I wouldn't call it ideal, namely because it requires a second window open when you might be starved for screen real estate and also because accidentally opening your mail once in a normal window could cause damage.
The Ghost Incognito extension allows you to force certain sites to always use Incognito mode. I realize using an extension to accomplish this objective is somewhat ironic, but seems like it'd work in this case if you can trust that one extension.
> Because Google Chrome does not control how extensions handle your personal data, all extensions have been disabled for incognito windows. You can reenable them individually in the extensions manager.
Keeping your gmail tab in an incognito window might be a good approach.