Gmail has added support for extensions
The extensions for Gmail will depend on the context, so they may or may not be displayed depending on the content of each particular email. For example, the Intuit extension allows users, like QuickBooks clients, to quickly and easily create receipts directly in Gmail. It is expected that in due course there will be more extensions, oriented not only to enterprises, but also to ordinary users.
Its important to note that there is as yet no separate store where you can download extensions for Gmail. Companies need to contact Google to get access to the features they need.