How to get GLMSystemWindow from GTPlayground

I wanted to write a small utility snippet involving GTPlayground. I want to transfer all my playgrounds from one image to another image. Something like

(GTPlayground allInstances reject: [ :each | each title = 'Playground' ]) do: [ :each |
        (each title asFileReference , #pg) writeStream
                nextPutAll: each entity content string;
                close ]

The snippet depends on the window title. I think I've found a bug because in the menu "Change title…" does not change the title of GTPlayground but of the GLMSystemWindow it is associated with.
I did not find the way to get the GLMSystemWindow from the GTPlayground in order to be able to access the title.

thanks,

Norbert