Oh, no? I contacted their support, and they do want to support it and have free options for you to test new updates. You do not have to “request” a demo, you can just go to https://www.gravityforms.com/demo/ and start that demo.
If they did not want to support your plugin, they would remove it from their website, and it is still there: https://www.gravityforms.com/community/mailpoet-3-integration/
Of course, if for some other reason, you no longer want to support this useful free plugin, you have no obligation whatsoever.
Until now, you said you did want to maintain it, but you could not, because of Gravity Forms not providing you a free license or a free option to test their latest versions.
It is clear to me they do want that add-on to be maintained.
It would be nice to have that clear once and for all for us thankful users that wait to get it updated?