I do tend to think LGW could be the most vulnerable infact of all the London airports. If, and I strongly emphasise "if", BA do go and "if" Norwegian or Virgin do so aswell, LGW potentially has a lot to lose. Not to mention the range of other long haul and full-service carriers that would likely shift to LHR where they could acquire appropriate slots.
On the other hand, I don't see the real benefit Ryanair, Jet2, Wizz etc. would have in shifting STN/LTN capacity down to LGW? After all nobody will be expanding to any significant level for the forseeable so would only be a switch, NOT in addition to. The only real longer-term gain one could argue is access to what would become extremely valueable slots when a fuller recovery comes about in 20XX, but they'll be no hurry in piling into LGW for that purpose alone. They'll each take a keen interest in any opportunity I'm sure but as ever with the LCCs, it's almost simply a matter of cost and operational performance (which LGW cannot reasonably boast too much in favour of STN and LTN to be totally frank). LGW may be able to offer some commercial terms they simply cannot refuse, but equally STN and LTN will be all to aware of the prospect of LGW trying to poach their bread and butter business if that were to come about.