There are three bosses that spawn in Hellgate. They are the Rotworm Queen, The Evil Eye, and Zevelon Duskbringer.
As far as we know The Evil Eye isn't worth killing. It doesn't drop any special items.
The Rotworm Queen can be found here. There are only Rotworms around so you could leave a low level character with a stealth ring and be able to logout immediately if it wasn't there. You should able to kill it at a low level as it isn't that strong. This is an easy boss and could be defeated by any vocation in the level 10-15 range if you were prepared.
The Evil Eye
The Evil Eye can be found to the north eastern part of the Bonelord area. You could leave a character in many places around here. This boss spawns with other Bonelords and Elder Bonelords. There could be lots of summons here to deal with. I would recommend at least level 40-50.
Zevelon Duskbringer is found right by the entrance to Hellgate when you take the teleporter from Ab'Dendriel. You can park a noob character there with a stealth ring and may be able to find an area where you can logout but there are Ghouls that could give you the logout block. There are also skeletons and demon skeletons in the area. Your best bet here is to just leave a low level character with stealth rings and let the boss kill it. Then run on your main character to kill the boss. This boss has 1400 health and can summon 3 vampires. You can end up taking around 1000 damage per turn. This would make it difficult to solo on a lower level knight. As a mage you could probably do it around level 50-60~ by running away and using gfb.