If GSM do pull-out, at this moment in time there is no-one else to step in, Ryanair's Boeing 737-800s are too big, except for the odd route, and Ryanair will realise that.
I think what the management would have to try and do (if they can), is attract several new routes from several LCCs, once Clickair expand a bit more we could be an attractive proposition for them, Ryanair could do several routes, perhaps they could get away with a 1 aircraft base, I don't know, Wizz could expand a lot too.
But we haven't crossed that bridge yet, so I'll shut-up.