You can never ever prove Glass-Steagall would have prevented the crisis, never. You can only look at the factors that created the crisis.
Banks were forced to grant mortgages to the non credit worthy because of the community reinvestment act. That's what started this ball rolling back in 1992. Not because of the repeal of Glass-Steagall in 1999.
Stop conjuring up this red herring, it does not exist.