The Great Demon Lord is the evil/demon alignment equivalent to the Demi God, which is a being that has surpassed the Hero. Although the existence of Demon Lords is much more abundant than the Hero, it is possible as, if not, more difficult to demon one because Demon Lords are prideful creatures and do not serve just about anyone.
Unlike the Demi God, whose ascension seems to be shrouded in mystery, the ascension to Great Demon Lord seems to be fairly simple: have several Demon Lord subordinates and undergo and evolution as a kind of 'refresh' for the Voice of the World to update the status. However, it is a little counter-intuitive of the VotW needing a 'refresh' to upgrade someone's Title and yet Rimuru being able to grant Titles unto his subordinates whenever he wants.
As such, it can also be said that the precise ascension to a Great Demon Lord is also somewhat mysterious.
Even if it sounds simple, it isn't so. If one needs to rule several Demon Lords under them, then that would mean that individual would need to have that level of strength to back it up. Otherwise, prideful existences like Demon Lords would never follow beings not worthy.
Although it is unknown exactly how many "several" refers to, it can be assumed that one needs to have at least more than 2 subordinates who are Demon Lord class.
A normal Demon Lord class individual is somewhere between EP: 200,000 ~ EP: 400,000. Footman and Tear are just below EP: 400,000 whereas Laplace is more than that (probably 1,000,000+).
However, Rimuru Tempest is a Great Demon Lord through and through. He has 14 subordinates having an EP: 1,000,000+ (9 of which have EP: 2,000,000). Though, among them, Diablo is the only being having EP:5,000,000+. Also, there are multitudes of subordinates having a power equal to a normal Demon Lord class.
Another thing to note is that the title to a Great Demon Lord is updated only when the Great Demon Lord "candidate" undergoes a great evolution that requires the acknowledgment of the Voice of the World (for example, obtaining an Ultimate Skill).
During and after the prize-giving ceremony due to the victory in the first battle against the Nasca Empire, Rimuru awakened 7 of his subordinates into True Demon Lords, however, he was still titled a Demon Lord according to the Voice of the World. Only after Raphael evolved into Ciel and upgraded all of Rimuru's skills (probably also coupled with his evolution into an Ultimate Slime sometime earlier), his title was updated to a Great Demon Lord.
Similarly, Kazaream had all his parameters increased when he was reincarnated as a Seraphim and also obtained the Ultimate Skill Dominion Lord Melchizedek before being recognized as a Great Demon Lord.
- Rimuru Tempest: Confirmed and undeniable.
- Kazaream: The only evidence is the Web Novel chapter: 203 in which he was stated to be "Great Demon Lord Kazaream" and he himself declared as such. It is yet unknown if he was recognized by the VotW to be a Great Demon Lord or not. But he was defeated by Benimaru.