gscatter() does not permit an axes to be passed in, and also does not appear to allow a 'Parent' property/value pair. There is no documented way to force gscatter() to draw on a particular axes.
You can get it to return the handles to the line objects and then you can set() the Parent property of the line objects to be your desired axes, but you would probably also be wanting to get the tick labels and so on that are generated, and you would need to find the legend-like object and take care of it. It might even be a legend; if so then before R2014b, it is in its own axes and so cannot be forced to be in a particular axes.
