I'm roleplaying an evil party and when Lothander gave me his gruff, I rebuffed him and he threatened me and walked away. So as he was walking away I attack and successfully killed him. This seemed to result in two things: he poisoned me anyways, and Marek doesn't ever appear to tell me about it, thus not opening to the quest to be healed. What I did to circumvent this circumstance was to use Gatekeeper to add Marek's Antidote to my inventory. Here's where it gets tricky. I have two PCs. I started a multi-player so I could have two custom characters, and the rest being NPCs. I gave the antidote to my first PC and it seemed to have worked. But then after a while my second PC was still getting sick, even when I gave him the same potion, so I got the idea to use gatekeeper again to give him another new marek's antidote. This didn't work and everyone dies! ARGH!!!!!!!!!!!!!!!!!!!!!!!!! Any way to hack this poison away, because all of my saves are right before the total death occurs?

Open up the ingame console and type this:
CLUAConsole:SetGlobal("partycured","global",1)
and then hit enter.

That should fix it, as the game now considers your party to be "cured."

Thanks! Fixed.

This is unrelated to the poison scenario, but I've downloaded BG1 in a new folder of my C: drive as opposed to program files and I've downloaded Gatekeeper's latest version (v1.09.02 I believe). I can't seem to get a proper save file location for Gatekeeper to work though, so I was wondering how you managed to get yours working.

I also had problems getting Gatekeeper to notice my Baldur's Gate 1 files. Maybe it doesn't work with the vanilla version? Anyway, I had the same problem with the poison quest and used the Sword Coast Keeper to bring the dead villains back into the game and get things going again. The Sword Coast Keeper is an editing program that is specifically designed for the first part of the series. Of course one can also use it to edit character stats and the likes.