I've never worked with googleplay, but if it's anything like other things in C2, it should be something like this:
First you will have to wait untill the request is done..
It's asynchronous so you should have a condition of on highscore request succes (or something like that)
Then you'd have to make the score appear (setting a text-object to googleplay.highscore?)
Not sure if this will work, but it seems like the normal way of doing things..