Gravity Forms does not dissociate deleted users from form entries. This code removes user IDs from entries when users are deleted.

remove-deleted-user-from-gform-entries.php

add_action( 'delete_user', 'remove_deleted_user_from_gform_entries' );

function remove_deleted_user_from_gform_entries( $user_id ) {
	//$user_id was just deleted, remove this from any entries if stored as created_by
	global $wpdb;
	$wpdb->query( $wpdb->prepare( "
		UPDATE 	`{$wpdb->prefix}gf_entry`
		SET 	`created_by` = NULL
		WHERE 	`created_by` = %d
	", $user_id ) );
}