There is no agronizing method for this AFAIK. Ig you really need it, you will likely have to code the base API of the SWT platform according to your GTK example. One way to do this, which builds / works everywhere, is to use reflection to invoke methods based on the return of SWT.getPlatform ().
If the window is launched with the mouse (for example, by hovering), you can use the mouse location as the base location for the hover.