If GBounds is undefined, then you need to define it before you attempt to use it.
Make sure a line something along the lines of
var GBounds = ...
exists where the ... is replaced by the appropriate identifier.
[Update for api v3]
I'm not entirely sure because I haven't tried it myself, but I found this answer on another site that might lead you in the right direction.
Clickey[
^]. I know that the answer is technically related to ColdFusion, but the javascript use of the API might be what you are looking for.