Reading back through this thread, it's funny to see how this was once one of those historic bits of Forteana that you really had to search hard to find information on, but where we've since had some decent work on it made available (followed almost inevitably by the usual deluge of poor quality writeups on commercial websites by non-Fortean writers desperate for 'content'). So I'd guess Gef is probably more widely known now than he has been at any time since the 30s.
It was only after the publication of Christopher Josiffe's excellent book on the Dalby Spook that I realised that, rather than the Irvings being a family with no prior links to the IoM before moving there from Liverpool, Margaret was in fact originally a local girl. This strikes me as being possibly important.
It seems apparent that at least one of the family was in some way responsible for Gef's 'creation' but the real question is whether it was Voirrey, Voirrey and her mother, or whether they were all in on it at some level.