I'm pretty sure the thing is like Deadalready said -you entered the city area with weapons ready, which will angry the polices. You have to remove your weapons from your hands and order everyone in your party to do the same. If those polices are already hostile it's possible you can't do anything. Try to remove the guns before entering the town and if they will still attack, you can't enter peacefully in town anymore.
If this will happen, you still can do every quests with some tricks
Train sneak. When you're good enough (about 70-100 should work) you can walk there without anyone noticing. You can still talk to important persons, even if they're hostile. Or then you can just kill the guards and save the important persons for quests
You definetely should use more than one save slot, because there're bugs which can prevent further playing. Saved gamefiles can also corrupt or some other unexcepted error could be occured and you have to restart the whole game. This can happen with any game, not just with Fallout2.
With the geck...hmm. I think there's no timelimit for returning the geck. I'm not 100% sure though.