Hi. My first thought was that the leading underscore in the custom field name would cause Gravity Forms to not see the hidden field. However, their old support forums said their plugin can see the fields that get hidden from the wp-admin custom field metabox.
Therefore, this support thread — https://www.gravityhelp.com/forums/topic/existing-custom-fields-not-being-recognized — might have the best advice: just manually create/enter the custom field name and then try out the form and everything should work as expected.
However, our plugin stores datetime data in a very specific format, so make sure that Gravity Forms is saving data in the format our plugin requires to work properly.
If you’re asking about Gravity Forms because you’re wanting front-end event submissions, we sell Community Events, which would make Gravity Forms unneeded (at least for this purpose).
Last but not least, we have a pretty great Refund Policy so you can buy one or many of our add-ons and thoroughly test them.
I hope this info helps. Let me know if you have any follow-up questions.