Triggering an email

Yes, something like this is possible. We'll need more details to help you with it though. A link to your form showing the field you're talking about will help us give you specific advice.

You would use a hook to run some code after each form submission. You could check the number of slots remaining, or number of people already registered, and when that condition reaches your threshold, you will send a notification email. The hook you could use is gform_after_submission and it's documented here:
http://www.gravityhelp.com/documentation/page/Gform_after_submission

If you need more help, please share specific details of what you're trying to accomplish. Thank you.

We're going to need a little more information from you first.

1. Is your form online now? If not, it will need to be, so we can create an example using your actual form inputs instead of hypotheticals.

That will probably answer a lot of questions. Please post a link to that form when you have it online, or export the form as XML and email it to chris@rocketgenius.com if the site is not live yet.

Thanks.