The reason that lead to this naming is due to both the characteristics of Gilgamesh and Saber.
Gilgamesh = Gold
Saber = Sword
"and" is used for better clarity in order to avoid misunderstandings. Since "Gold Sword" and "Golden Sword" may refer to weapons used in games like The Legend of Zelda or Final Fantasy.
This article may contain spoilers
In both Fate/stay night and Fate/Zero, Gilgamesh had many times attempt to propose to Saber (this can be seen especially in Fate route in Fate/stay night), usually with demeaning and insulting words.
However, Saber has no feelings for him, added with another reason where Saber is supposed one of the heroines for Fate/stay night (which means she is designed specifically for Shirou Emiya to score her). Hence, this pairing is almost entirely one sided.
Initially the shipping was not that famous until the airing of Fate/Zero first season that the popularity was then picked up, forming this couple.