A couple things. You need to add culture to your query. Node can have multiple documents (one per each language), so no need for transformation - nodeGuid and culture will identify a unique document. Secondly Where returns InfoObjectCollection - use FirstItem instead. You do not need WithAllData if you are looking for common document properties like RelativeURL (WithAllData - might create an excessive SQL query):
{% Documents.Where("NodeGUID = '" + CurrentDocument.NodeGUID + "' and documentCulture='" + CurrentDocument.DocumentCulture + "'").FirstItem.RelativeURL|(identity)GlobalAdministrator%} to check if your are getting it correctly.