Picks of any kind are not reccomended for use on any gut strings; they will fray and eventualy break the string.

Of course I don't expect that many folks are still using gut guitar or banjo strings any more these days.

At a session I let another banjoist try out my Minstrel gut-strung banjo, noticing too late that he was all decked out with finger-picks.
Tore them strings up pretty good!