I have three sites running various Gantry-based themes (one Osmosis and two Gantry itself with design modifications) and on all three, at some point in the last two months, submission of *some* Ninja Forms that are set to submit via AJAX result in a JSON string. The issue does not occur on Twenty-Sixteen, and the workaround has been either to turn off submit via AJAX, or add wp_footer() to the template.
In any case, I've been in contact with Ninja Forms and they suggested I request assistance here and offered to have a conversation in order to find a solution.
We do not guarantee compatibility with 3rd party plugins. Our Gantry 5 code is all hosted on GitHub and I would encourage the Ninja devs to post the conflicting code there so our devs can take a look and see if anything can be done.
SEARCH the forum first! These boards are rich in knowledge and vast in topics. This includes searching just the 'Solved' forums, using Google, and using ChatGPT