All of the google (game) api's have two stages. First is to setup the request, then you need to execute.
Your code should look something like:
var json= { leaderboardId: 'dlsfhqo3irhq', collection: 'PUBLIC', timeSpan:'ALL_TIME'};
var request = gapi.client.games.scores.listWindow(json);
request.execute(function(response) {
// do something
});