Hi all i am a long time Type Moon Fan. The Fate Series has stolen my heart and as such i am a big Conspiracy nut and like to put my own spins on the established Rules set by the anime and visual novels.
My all time favorite Servant is Gilgamesh and as such I would like to propose a theory that has gotten to the point me and my friends can no longer decide a winner and leave it up to you on this Forum. I ask you to feel free to criticize and support the theory i want to know how plausible it is. Message me or comment and i will attempt to protect and validate my Crazy Theory. So without further Ado I present you with a Theory- Can Gilgamesh Summon Himself
Sporting points- Points to consider before diving into the theory-
1 We can assume that all/ most of the servants have some level of magical ability. This is true due to their Noble Phantasms being in their own right Magic. This is supported by Saber's Admiration for Assassin (Stay Night) When he Preforms his Noble Phantasm without Magic. Due due Saber's surprise at this fact we can assume that Noble Phantasms are magic in nature and require magic to use.
2 Archer's (stay night) identity is Heroic Spirit Emiya/ Future Shiro. Due to this it is established that Heroes can be summoned from any point in time regardless of any Paradox.
3 Caster (stay night) is able to summon Assassin. Establishing that Servants can summon other Servants Given enough Magical Prowess.
4 Fate Extra CCC takes Place after the events of Stay night and Gilgamesh is once again summoned
The Theory-
Based on these Facts Proven by events in the Fate Series It is in fact possible for Gilgamesh to summon himself in another Grail War.
Gilgamesh- Given second life by the Grail in Zero after becoming exposed to the "mud" produced by the Grail before its subsequent destruction at the hands of Saber. Due to this Gilgamesh is no longer a Spirit but a living being. In Fate extra CCC Gilgamesh is summoned once again meaning that even if given life a Spirit can be put back into the "drawing" to be summoned. Also in accordance with Fate Stay Night Servants can be summoned even if an alternate form exists. In a future Holy Grail war Gilgamesh if a strong enough Magi (given that all Spirits have some latent Magical ability) Given that Gilgamesh was strong enough, or that the pool of contestants was small, or the majority of the magi were weak, Gilgamesh could be chosen (as when caster was able to summon Assassin this was due to the poor quality of mages and her own ability) by the Grail and able to Participate in the grail war. Gilgamesh then would be tasked with the summoning. Given the knowledge presented by the Grail and haven been summoned, he will have already had knowledge of the process. By using artifacts from the Gate of Babylon and possibly his own blood he would have no problem establishing a link with his Servant self, and due to his ability to be summoned in multiple classes Gilgamesh could fill multiple slots even if Archer was taken (given that Tohsaka stated that he was surprised that Gilgamesh was summoned in archer and not saber due to his prowess in close combat) Once again as in Stay night with Archer and Shiro Gilgamesh is a Physical being separate now from his servant form he can once again be summoned even by himself!!!
Follow Up- Answering Doubtful friends Critiques-
While yes Gilgamesh is Dead after the events of Stay Night, I simply refereed to this as an example simply attempting to convey that Gilgamesh could have been chosen in Stay night, or in another Grail war if he was again given second life.
While Caster killed assassins master she still herself summon Assassin and used the temple as a Catalyst in order to summon him. And while this may be the case Gilgamesh has been reborn in this scenario and does not fall under the Classification of Servant and as such does not have the same disadvantage.
This is also assuming that Glgamesh Could be chosen By the Grail. This is possible as Shiro was technically a Heroic Spirit and was still able to be selected as a master.
A question has been raised that not being a high ranking Mage Gilgamesh would not be able to support summoning a servant. I argue that he is more than suitable enough, Glgameshn possesses a noble Phantasm and in their own right are magic in nature. The fact that Gilgamesh is able to use the Gate of Babylon multiple times after the Grail war, where he and Kotomine's link was severed (as evident by the removal of his Command Seals) and he no longer supported him with Mana. He is also 2/3 Divine and possessed Immediate Action Rank A (the ability to survive without the aid of a master or mana supply, relying solely on his own Mana) and this should be more than enough to allow for him to call forth a servant. (and arguably allow his servant self to survive even if his own mana is not enough.
The Final Question one might have is Why would you think this up in the first place??? The reason is simple iI over analyze way to much but mainly because Gilgamesh is so amazing how could the world not be any better with two of them!