In your opinion, what is the main reason that made Joe Gillis to stay with Norma that long and continue in that fake situation despite his repulsion and interior wish to leave many times?
At one point I got the idea that he has a deep sympathy towards Norma and her mental state and wants to help her, but then again was it his greed and need for money that forced him to continue living with her?
Both -- at different times. First, it was survival in Hollywood (not greed), later it was a combination of survival and caring (sympathy). I think the closet full of suits, gold cigarette case, etc. played a very small part in the dynamic. He used that as one piece of evidence for his own self-loathing, as they had become building blocks in the wall separating him from his writing and the woman he wanted to spend the future with. The other important piece of evidence was his betrayal, however unintended, of his best friend.
shareIt's quite a critical observation that Norma only gave him expensive clothes, wine and items. She never gave him a new car or actual money because those things would increase his freedom.
shareA mixture of both. He clearly liked the money, coming from indigence. See the way he swims in the pool, dresses sharply, and just overall enjoys the luxury.
But he is also sympathetic, probably beginning around the point when Norma tries to kill her self. See the way he ran out of the party when Max tells him over the phone that Norma slit her wrists. He felt a little sorry.
He eventually ditches and tells Norma the truth about her fan mail. Those actors occurred cause he was upset about how Norma treated Betty. The incident with Betty removed whatever sympathy he had towards Norma
IMHO the 3rd and largest factor was that Joe was running away from his own life, he was hiding out there to escape his creditors and to avoid thinking about his own failures. I think the desire to hide from jus reality was stronger than his greed or his sympathy for Norma, because both Norma and her presents made him uncomfortable.
And it was Betty that got him out of that rut, she got him writing and thinking of the future again (and he mistook that for loce), after all those weeks of drifting along as Norma's escort and lap dog.