Are all of the Holy Grail War servants based on real heroes? Saber's character in Fate/stay night and Fate/zero is based on the real-life hero King Arthur. Are the other Holy Grail War servants also based on real-life heroes? And if they are, who are those heroes?
For the most part, yes, but there are some exceptions. I assume you haven't read the Fate/stay night visual novel, which answers your question. As such, spoilers for that below.


   Some of the heroes are basically just reinterpretations of people known to have actually existed in the historical record. These are F/z's Rider (Alexander the Great), Caster (Gilles de Rais), Archer (Gilgamesh), Assassin (Hassan-i Sabbah); and F/sn's Assassin (Sasaki Kojirou; or at least, I think the consensus is that he was real) and True Assassin (Hassan-i Sabbah).

 Other heroes are reinterpretations of fictional heroes, or of people of dubious historicity. In the world of Fate, these people are treated as having been actual historical figures who just happen to be known in the modern day through fictional writings. These include F/z's Saber (King Arthur), Lancer (Diarmuid Ua Duibhne), Berserker (Lancelot); and F/sn's Rider (Medusa), Saber (King Arthur again), Lancer (Cú Chulainn), Caster (Medea), and Berserker (Heracles)

 And then you have the one huge exception, which is F/sn's Archer (that is, Rin's servant, not Gilgamesh). F/sn's Archer is actually Emiya Shirou, who is fictional.

 There's also a delicate issue surrounding the various Hassans who are summoned as Assassins - the idea there is that all ordinary Assassins (not including weird exceptions like Sasaki Kojirou) correspond to a leader of the historical Assassin cult. The issue there is that there are basically no records (in the real world) of what the leaders of the cult were like, so their personalities and abilities in the world of Fate are entirely an invention of the authors (Urobuchi Gen and Nasu Kinoko for F/z and F/sn respectively).