This hooks validates the Gravity Forms form submissions checking if the user, has already been registered in the database.

gf_validate_register.php

<?php

// remember, the hook suffix, should contain the form id!
add_filter( "gform_field_validation_3", 'gf_custom_validation', 10, 4 );

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

	$classes = explode( ' ', $field['cssClass'] );

	// lets check if the user with such a username is already in the database
	if ( in_array( 'username', $classes ) ) {

		$user_id = username_exists( $value );
		if ( $user_id ) {
			// let's mark this field is invalid
			$result['is_valid'] = false;
			$result['message'] = 'This username is already taken.';
		} else {
			// let's mark this field as valid
			$result['is_valid'] = true;
		}
	}

	// let's check if the user with such an email has already been created
	if ( in_array( 'email', $classes ) ) {
		
		if ( email_exists( $value ) != false ) {
			// let's mark this field as invalid
			$result['message'] = 'This email is already taken.';
			$result['is_valid'] = false;
		} else {
			// let's mark this field as valid
			$result['is_valid'] = true;
		}
	}
	return $result;
}