I have a CD with information for the TSST (id3v2.3) field.
And I have defined a new column:

{Column gmb-art_discname}
title = _"CD Name"
menutitle = _"gmb-art CD-Name"
text: text(markup='<small>'.$discname.'</small>')

But the column is blank.

I've just tested it, it works for me.
Have you activated the field in the preferences' fields tab (and restarted) ?
After activating the field you need to refresh the tags, right click on the song, choose re-read tags.
Then the value should show up in the tag edition dialog. If it doesn't, maybe there is something wrong with the tag, send one file to me by email if that is the case.

I had forgotten to activate the field. Thank you.