Sample code for creating custom Episerver Forms form container block. This is the sample avaScript to get the data-* attribute values when form is submitted.

gtm-script.js

if (typeof $$epiforms !== 'undefined') {
  $$epiforms(document).ready(function myfunction() {
    $$epiforms(".EPiServerForms").on("formsSubmitted", function (event) {
      if (event.isFinalizedSubmission && event.isSuccess) {
        // we have our custom dataLayer object, we could also have this code somewhere: window.dataLayer = window.dataLayer || []; and then use that
        if (typeof dataLayer !== 'undefined') {
          var currentForm = $$epiforms(this).get(0);
          // using dom native methods, to support wider range of browsers that don't support the 'dataset'
          var categoryName = currentForm.getAttribute("data-gtm-formcategory");
          var formName = currentForm.getAttribute("data-gtm-formname");

          console.log('Form submitted, GTM dataLayer values, category name:[' + categoryName + '] form name:[' + formName + ']');

          dataLayer.push({
            'event': 'formSubmission',
            'formCategory': categoryName,
            'formName': formName
          });
        }
      }
    });
  });
}