Ninja Forms Google Tag Manager Trigger

nf-gtm.js

<script>
jQuery( document ).on( 'nfFormReady', function() {
  nfRadio.channel('forms').on('submit:response', function(form) {
    window.dataLayer = window.dataLayer||[];
    dataLayer.push({
      event: 'ninjaFormSubmission',
      eventData: form.data,
      formID: form.data.form_id,
      formTitle: form.data.settings.title
    })
  })
} )
</script>

I found this works best if you're using a DOM Ready trigger in GTM, rather than a Page View one, as GTM may load before jQuery and it simply won't fire the event.

Correct me if I am wrong but doesn't this fire for each form element?

With multiple forms on a single page, the current implementation would fire 2x data pushes. It's better to use

jQuery( document ).one( 'nfFormReady', function() {
...
}

Then the data push only happens once on any form submit on the page.