Agents are non-technical people, specifically employed to, as you put it, "trot out scripted answers". So, trotting out scripted answers does not make the agents incompetent, it merely means somebody at a higher level needs to recheck whether their scripts are giving the correct information and to revise them if not.
A review at a higher level should normally be invoked by using the complaints procedure https://giffgaff.com/support/complaints -- which should presumably operate on three levels:
Much of the discussion in this thread is not about whether the agents gave the correct answer according to giffgaff's implemetation of EU roaming, but about whether the giffgaff rules comply with the EU rules. That ought to be raised as a separate issue -- it's nothing to do with the "competence" of the agents and if anything needs to change, it's giffgaff's rules and/or documentation, rather than the behaviour of the agents.