Certain things like hover tools do not work for image_url objects of a plot, and as the GMap version you are using is a subclass of Plot, I suspect similar limitations exist with it and other hit-test behaviors.
In the past when I needed a feature that was not available via ImageURL, I plotted co-located images with overlaid elementary glyphs like a circle. Then set the alpha attributes of the elementary glyph to 0.0 so that they are transparent to the user but allow interactive behaviors to seem associated with the images.