How to use the Marketo forms events in GTM?

The pattern is the same as when using GA to log successful form submissions, only with GTM you'd use eventCallback where GA uses hitCallback.

MktoForms2.whenReady(function(form){
  form.onSuccess(function(vals,tyURL){
    dataLayer.push({
      'event' : 'formSubmit',
      'eventCallback' : function() {
        document.location.href = tyURL;
      },
      'eventTimeout' : 3000
    });
    return false;
  });
});