Unfortunately, "widget” is such a overused word that it’s hard to know what it means any more, especially in the Mac arena.
Geektool is definitely not a widget, nor are the things that are built with it (as you already know). It does work on Catalina, although it has a few quirks (actually it always has had a few quirks).
Uebersicht, unfortunately, is one of the over-users of the term “widget” but AFAIK it also works on Catalina. I’ve never used it myself but I’ve seen it mentioned more than once. Did you try searching the forum? This thread looks promising.