As far as i'm aware GLA is a 757 only base (2 aircraft) so you would have to be put onto the 757 to be in with a shot. As far as your chances of getting GLA after that, to be honest, i really don't know the answer to that. I do know that the guys on the 757 mostly seem to be determined to stay on the aircraft until they are moved off it due to fleet replacement etc (737 max from 2018). For the GLA guys this would mean a base move if they wish to change types which might not suit if Glasgow or nearby is home. One could argue that EDI is there as an option but then there is only 1 737 based there, again i have no idea what the chances are but it seems the larger bases have the greater movement due to the fleet sizes within those bases.