It has potential for RP friction between factions - but also potential friction within the GMG itself.
I would like to work out some 'backdoor' diplomacy - whereby Houses such as Bretonia are officially aghast at such a move, but can arrange for repatriation of their people on the side (maybe with Zoners as an intermediary).
Something does need to be worked out - if the GMG shows up with Bretonian prisoners and they are turned away - the Guild ships would likely be targeted by the Corsairs or Hogosha for their cargo...