Why look for a Germanic etymology?
My Grosser Historischer Weltatlas identifies Guthalus with the
Niemen or Memel. That's maybe just a guess, assuming Pliny is
listing rivers east to west, but seems a reasonable one. This is
getting close to Baltic territory, but anyway isn't the game with the
major rivers of central Europe "Alteuropaische hydronymik",
looking for a vague I.-E. meaning but pre- any known languages.
Or am I forty or fifty years out of date?