breaking bad - Why exactly did Walt leave Gray Matter?

According to show creator Vince Gilligan and co-showrunner Peter Gould in a new interview with The Huffington Post:

The real reason White left Gray Matter, which serves as part of the
motivation for the character’s dark turn, has remained a mystery, but
while chatting with The Huffington Post about “Yo Soy Saul,” an
original song created for “Better Call Saul,” show creator Vince Gilligan and co-showrunner Peter Gould also
finally opened up about one of the biggest unanswered questions from
the series:

“Breaking Bad” actress Jessica Hecht, who plays Gretchen, mentioned in
an AMC Q&A that Walt left the company and their relationship because
he felt inferior. Gilligan confirmed this was true to HuffPost,
saying, “She’s correct, and that’s what I explained to her and to
[Bryan Cranston] before they shot that big scene between the two of
them where they were at the restaurant.”

The scene Gilligan is referring to takes place in the Season 2 episode
“Peekaboo.”

“It ends with him being so nasty to her saying, ‘Fuck you,’ and then
she leaves tearfully,” said Gilligan. “In my mind, the interesting
thing here — and I always kind of hate to nail it down so explicitly —
but let’s put it this way, most viewers of ‘Breaking Bad’ assume
Gretchen and Elliott are the bad guys, and they assume that Walt got
ripped off by them, got ill used by them, and I never actually saw it
that way.”

Gilligan explained that the truth is more nuanced. It all stemmed from
White’s feeling of inferiority while spending time with Gretchen’s
family.

“I think it was kind of situation where he didn’t realize the girl he
was about to marry was so very wealthy and came from such a prominent
family, and it kind of blew his mind and made him feel inferior and he
overreacted. He just kind of checked out. I think there is that whole
other side to the story, and it can be gleaned. This isn’t really the
CliffsNotes version so much. These facts can be gleaned if you watch
some of these scenes really closely enough, and you watch them without
too much of an overriding bias toward Walt and against Gretchen and
Elliott,” said Gilligan.

Gould added, “I think the interesting thing is not exactly what
happened but the fact that Walt hasn’t let it go over all these years.
He has no perspective on himself. He gets to the point where all he
can really do is try to justify everything that he’s done.”