Gmail is getting third-party support for add-ons later this year, as Google has just introduced the new capability for their email client today. While G Suite users won't immediately be able to take advantage of the new add-on support as standard, Google has also introduced the developer preview of this feature to give developers a chance to test out creating a third-party add-on. While there will likely end up being multiple third-party add-ons available once they officially launch, and even more once the feature has been live for a little while, Google has already worked with a handful of different businesses to test the third-party add-on support out within Gmail.
Intuit was one of these companies. If the name sounds familiar that's because Intuit is the company behind QuickBooks. Imagine having the QuickBooks add-on integrated with Gmail so you could more easily create an invoice to send to people (or customers) and with little effort add it right into your email without ever having to leave Gmail in the first place. In addition to Intuit, Google also worked with Salesforce and PropserWorks to integrate add-ons for their services as well, all of which developers who sign up for the preview should also get access to for testing.
Third-party add-ons will definitely be a useful functionality for everyday Gmail users, but only if developers are creating the content. The nice thing about what Google has done with third-party add-on support for the developer side of things is that they have made it ridiculously seamless for the development process when it comes to where the add-ons work. For developers who create an add-on, it only has to be written once and Google notes that it will work across the web, Android, and iOS platforms from the start, so they won't have to write multiple add-ons to accommodate all users. This will save developers time and should get more developers interested as they won't have to do as much work to reach a larger number of users. At this point in time Google hasn't mentioned an exact date or even a time frame other than "later this year" for when third-party add-on support will go live, but they did say that they expect to start sending out early access invitations to developers in the near future for those that show interest through applying.