Walt wasn’t (really) being petty about Gale’s work. The pettiness about Gale’s work was actually just a cover, so that Walt could bring Jesse back, because he wanted Gus to think that Jesse was indispensable to his work (if I’m not mistaken, to keep Jesse alive, because Gus viewed Jesse as an unpredictable threat).
Because Walt kept fucking with his business, and drawing too much attention. You’ll recall that he killed two of Gus’ dealers in order to protect Jesse. Which is normally something that Gus would kill him for, except he couldn’t because he was indispensable. So much of everything going sour with Gus was about protecting Jesse, really.
