If I am not wrong GAIL was forced to pick up subsidy tab for petrol/diesel in past. Not sure about GSPL. I assume that GSPL being a state govt PSU, they were not obliged to share subsidy burden of central government.
Other advantage of GSPL is that a majority of valuation comes from Gujarat gas which is a well diversified and solid franchisee of gas distribution, unlike other gas distribution companies which are dependent mainly on auto/domestic consumers. Gujarat gas is run more professionally as it has its pedigree in British Gas.