Access to Dark Knight.

And I am not referring to Bat-Man. The Dark Knight I am referring to, is none other than that of the job skills that Gafgarion utilized.
To make this Job available on the job select menu, complete the following instructions as specified;

To begin with, this can only be achieved during Chapter II, around the chronological period that Gafgarion is a temporary member of your party.
Before his treacherous betrayal to you, and your small militia, unequip all of his armor. Every last remaining piece( This is a good idea in any case, as he cannot retaliate with his Dark Sword commands without his sword, and will simply punch at you, doing minimal damage. Whatsmore, you will be reducing his defenses, lowering his HP, and making him a much easier kill.).
Now, equip that armor on to someone else, and switch their job class to a priest.
Make sure that they participate in the battle against Gafgarion, and that they themself deliver the finishing blow, otherwise, this will not work. After the battle, "Dark Knight" should readily become available to you on the Job screen.

NOTE: While your job class is that of the priest, make sure you have the knight's "Equip Sword" ability on the character you wish to become a Dark Knight.
Also, this job class may, or may not become available to initial characters such as Ramza, Mustadio, or anyone else, I am uncertain because I have never tried this.
You also might want to consider making your subject one of the lacky's in Agrias team formation that join along with you. Anyone of them will do, male or female.

If anyone can recall the name of the battlefield where the battle with Gafgarion takes place, please let me know so that I can amend this post. Thanks.

Is this actually possible, or do I need to get General Leo and Sephiroth on my team first?

Please don't spread misinformation.