To track the submission of gravity forms and generate an event through Google Tag Manager, you can utilize the following code, It adds the onclick event to the submit button of the gravity form. So, whenever the submit button is clicked an event is sent to Google Server. You need to do two things to use this code

gravity_form_gtm_tracking.php

function add_conversion_tracking_code($button, $form) {
	$dom = new DOMDocument();
	$dom->loadHTML($button);
	$input = $dom->getElementsByTagName('input')->item(0);
	if ($input->hasAttribute('onclick')) {
		$input->setAttribute("onclick","ga('send', 'event', { eventCategory: 'GTM_Category', eventAction: 'GTM_Action', eventLabel: 'GTM_Label'});".$input->getAttribute("onclick"));
	} else {
		$input->setAttribute("onclick","ga('send', 'event', { eventCategory: 'GTM_Category', eventAction: 'GTM_Action', eventLabel: 'GTM_Label'});");
	}
	return $dom->saveHtml();
}
add_filter( 'gform_submit_button_1', 'add_conversion_tracking_code', 10, 2);