Full Code

full.php

<?php

add_filter( 'gform_field_input', 'gf_field_input', 10, 5 );

wp_enqueue_script( 'jquery-ui-datepicker' );

/**
 * Overwrite gravity forms input field
 * 
 * @hook gform_field_input
 */
function gf_field_input($input, $field, $value, $lead_id, $form_id) {

	// we don't want to change the admin look behavior of the form field
	if ( IS_ADMIN )
		return $input;

	// lets create custom form html in case it had the css class assigned
	if ( $field['cssClass'] === 'gf-date-picker' ) {

		// get the global post values for use with the get_field function
		global $post;
		$id = $post->ID;

		// have we set any values for the location and date field
		if ( get_field( 'loc_date', $id ) ) {
			// lets star the output buffering
			ob_start();
			gf_generate_datepicker( $field['id'], $value );
			// lets get the output 
			$input = ob_get_clean();
		}
	}

	return $input;
}

/**
 * Generate datepicker custom field
 * 
 * @param int $id
 * @param string $value
 */
function gf_generate_datepicker($id, $value) {
	// the value here is used for the pagination
	?>
	<input type="text" name="input_<?php echo $id; ?>" class="datepicker" value="<?php echo $value; ?>" />
	<?php
}

add_action( 'wp_head', 'gf_trigger_picker' );

function gf_trigger_picker() {
	global $post;
	$id = $post->ID;
	?>
	<script>
		jQuery(document).ready(function($) {
	<?php if ( get_field( 'loc_date', $id ) ): ?>

				availableDates = function(date) {

					// create the proper format for comparison
					var dmy = date.getFullYear() + '' + ('0' + (date.getMonth() + 1)).slice(-2) + '' + ('0' + date.getDate()).slice(-2);

					// check what date is available
					if (dmy === '<?php echo get_field( 'loc_date', $id ); ?>') {
						return [true, "available", "Available"];
					} else {
						return [false, "", "unAvailable"];
					}
				};

				$(".datepicker").datepicker(
						{
							// lets make sure that the dates are limited to the values of our ACF field
							beforeShowDay: availableDates
						}
				);
	<?php endif; ?>
		});
	</script>
	<?php
}

	
add_action( 'gform_pre_submission_1', 'gf_form_pre_submission' );

/**
 * Presubmission form handler
 * 
 * @hook gform_pre_submission
 */
function gf_form_pre_submission($form) {

	// iterate over the fields to obtain specific field's id
	foreach ( $form['fields'] as $field ) {

		// check if the date picker class has been added
		if ( $field['cssClass'] == 'gf-date-picker' ) {

			// get the field id
			$id = $field['id'];

			// do whatever you want with the values here (change the date format, etc.)
			$_POST['input_' . $id] = $_POST['input_' . $id];
		}
	}
}

add_filter( "gform_field_validation_1", 'gf_custom_validation', 10, 4 );

/**
 * Presubmission form handler
 * 
 * @hook gform_pre_submission
 */

/**
 * Custom GF validation function used for pagination and required fields
 *
 * @return string
 */
function gf_custom_validation($result, $value, $form, $field) {

	// lets validate only our custom field
	if ( $field['cssClass'] === 'gf-date-picker' ) {

		// let's obtain separate strings for each part of the date
		$value = array_map( 'intval', explode( '/', $value ) );
		
		// if the field is not valid then return the user
		if ( !checkdate( $value[0], $value[1], $value[2] ) ) {
			
			// add error message to the end user
			$result["is_valid"] = false;
			$result["message"] = "Wrong date, please check the format and make sure it's a valid one. ";
			
		} else {
			$result["is_valid"] = true;
		}
	}

	return $result;
}