Did Gilgamesh know about Sakura? Early into Fate/Stay Night, there is a scene where Shirou sees Sakura and another man:



In the Unlimited Blade Works Route, we confirm this other man is Gilgamesh. When Sakura talks to Shirou, she says that the other man was a foreigner and she couldn't understand him. However, we can see from the image that that may not be the case.

Since Gilgamesh has been tainted by Angra Mainyu's corruption of the Holy Grail from the previous war, and Sakura is a "Black Grail" Vessel with a deeper connection to Angra Mainyu than the "White Grail" Vessel (Ilya), did Gilgamesh already know about Sakura?
To complement Anon153's answer, here's a relevant quotation from TYPE-MOON Wiki:


  Gilgamesh is one of the antagonists in each scenario of Fate/stay
  night. He first appears before the start of the Fifth Holy Grail War
  around Sakura's house. He identifies her as “the other Grail” and
  comes to tell her “Go die right now.” It is just a suggestion rather
  than a command, as he believes that the matter will develop into some
  sort of entertainment regardless of the situation. When questioned
  about the encounter, Sakura just claims that he was asking for
  directions and that “It was English” in order to hide the truth.


And source referenced is as follows (Citation Note 14):


  Fate/complete material III World material - FAQ with Nasu: General Questions about Fate, p.130 
  Q: Prior to the start of the Holy Grail War, what was Gilgamesh doing around Sakura’s house? 
  A: Yes. He came to tell Sakura to “Go die right now”. It is rather a suggestion than command. Regardless the situation, from the King’s perspective, this matter will develop into some sort of entertainment. As a result, what Gilgamesh said was not a command. Sakura’s reply “It was English” was a convenient excuse to hide the truth.