Skeezix told me that "Gargoyle is really just a 'pack' of other interpreters modified to use the Gargoyle framework". Seems like Gargoyle uses Frotz as its zmachine engine and thus you probably do not need the Frotz-pnd if you got Gargoyle, which also looks nicer in the typesetting.

Thanks. According to the homepage http://ccxvii.net/gargoyle/ this also runs frotz-formats. Is there a reason to still keep frotz (as maybe it's better at some parts of interpreting)?