Suspect many of the non type-rated 'volunteers' are taking advantage of the early 744 rating from BA, (i.e. mainline 744 job out of seniority), with absolutely no intention of going on to work for GSS when their time comes.
As for the few, (some surprisingly senior), 744 rated guys putting their name forward I believe most are 'no-hopers' as far as LHS suitability is concerned. Remember the interviews involved for selection!
Apologies to any well intentioned non type-rated and qualified peeps genuinely going for it. Commiserations to the GSS FO's being affected. Shame on BALPA for signing up for the deal!