Let's put things in perspective.
Iskander was so charismatic that many soldiers pledged eternal loyalty to him - even after death. He was respected even by his enemies and loved by his people, and even the peoples he conquered grew to appreciate him.
Gilgamesh was such an arrogant fuckass that he was almost negating his CHA bonus. However, keep in mind that Enkidu - a man of equal strength to Gilgamesh, created by the gods specifically to put Gil in his place - ended up swearing his friendship and loyalty to Gil. Even the gods (Ishtar, to be specific) wanted to be with Gil, and those that didn't, they at least respected him greatly.
Even now, those that are enemies to Gilgamesh stand in awe of him.