This represents Fate/Stay Night episode 22, and is thus a continuation of "Reversed Causality".

Kotomine Kirei is a (crooked) priest who works for an agency that moderates the Grail Wars. However, he also fought in the previous Grail War, along with a man named Kiritsugu Emiya (who controlled Saber at the time). At the end of the previous War, when the Holy Grail manifested itself, Kiritsugu told Saber to destroy the Grail so it couldn't be used by the evil Kotomine. The Grail's destruction caused a huge fire that young orphan Shirou was caught in...but Kiritsugu saved him using Saber's magic scabbard. Once the War ended, Saber returned to the place from whence she was summoned (without knowing where her scabbard had ended up), and Kiritsugu became Shirou's foster father.

A Master will generally summon a Servant with whom they share ideals. Thus, Kotomine's Servant during the previous War was the equally sadistic Gilgamesh. Similarly, since Shirou was raised by do-gooder Kiritsugu, it only stood to reason that Shirou would summon do-gooder Saber in the current War.

As Shirou's friend Rin hastily explains at the beginning of this comic, early on in the current War, Kotomine stole control of Lancer after killing his original Master. However, Kotomine never mentioned to his stolen Servant that Gilgamesh didn't go away after the previous War ended. As you can see, Lancer isn't too thrilled to find out that Kotomine has been hiding a second Servant. Maybe he's jealous? In any event, Lancer promptly turns against Gil, and Gil promptly turns Lancer into dogmeat.

I'm of the opinion that Saber and Shirou should have given Lancer a hand. For you see, Gilgamesh is no joke. He has a nigh-infinite collection of legendary weapons, which he keeps behind his Gates of Babylon (the drill-like weapon that he uses to kill Lancer here is the legendary sword Caladbolg, against which Lancer has been destined to lose). He can convert the weapons into energy and launch them at his opponents by the hundreds. And he has Enkidu, the Chains of Heaven, which can restrain even a slippery Servant like Lancer. Basically, he's got a tool for every situation.

The word "gar" has come to mean "a man-crush on a male character who's so badass that he makes you feel like a little sissy girl". One can be "gar" for Lancer, Gilgamesh, or Archer (but we'll meet him later).