Bagso - Gatwick's Achilles heel has always been the fact that its airlines are simply not prepared to pay the increased airport charges that would result from a second runway. So you are absolutely right to question its viability.
In contrast, although Heathrow's charges are already higher and would rise further in the event of R3 going ahead, its airlines are likely to (reluctantly) accept the higher charges because of the overwhelming advantages of Heathrow (much higher fare yields etc).
To a large extent, GAL's campaign for R2 is a cynical exercise by its key shareholder GIP to boost its value ahead of a planned sell down of its shareholding within the next couple of years.