There's a good chance that adding the ability for players to withdraw money from a gang's bank would be abused or lead to problems in some way. I can just picture someone (who's in a gang) getting mad, emptying the gang's bank and then leaving the gang.
One way it might work is with strict limits, such as being able to withdraw $100,000, but making that the maximum within a 24 hour period, for example. This could come in handy in the event someone in the gang is in urgent need of cash, but similarly earning money on SFSE is a fairly quick process anyway.