[ZFR]

OK, that explains it.

Had you gone to the warehouse before killing Gulan, you'd have met Drawl just beside Walter's cage. Drawl is the leader of the sword coast gang, whom you must've heard of (Walter was one of them). Killing Drawl opens the electrified way into the crypts.
Apparently Drawl disappears after you kill Gulan (?? can't say for sure, because I never tried it).

If you have a save right before killing Gulan, you might want to reload it and go kill Drawl first (and finish off all the other beggar's nest quests if you've any left), then come back and kill Gulan. Drawl is worth a fair amount of XP and possibly carries a few items as well. It's like a mini-boss fight.