We develop Gmail Extensions and recently saw and applied to be included in the proposed Gmail Add-ons Developer Preview...
We have not had a Response from Google.
Has anybody been accepted into the Preview?
All I can find on the web is the following
https://gsuite-developers.googleblog.com/2017/03/bring-power-of-your-apps-into-gmail_9.html
Thanks for your feedback Paul C
There is still no definite date as to when the Invitations will be sent out. If you have filled out the form, next thing to do is to simply wait for the announcement/email.
As also stated in the form:
Please be advised that space in this program is limited. Applications will be reviewed on a one-on-one basis.
It's either still being reviewed or is already processed and just waiting for the invites. I suggest following G Suite Developers Twitter account (seen in the page) to be aware of any updates. I think if they would announce something like that if it comes live. Cheers!