ترجمة - إيطاليّ -لاتيني - il comandante mostrò grande clemenza nel punire i...

in desertorum puniendo --> in desertoribus puniendis

When gerundive has a complement (here in genitive) and is constructed with a preposition, it must be replaced by a verbal adjective. (One of the nastier rules of Latin grammar...)

You right of course as usual. Thanks a lot.