How does Walt know that Gus intends to kill him?

Question

In Breaking Bad, season 3, after Walt kills the two dealers who murdered Tomas, he drives out into the desert where he meets Mike who then escorts him back to Gus's car. He suggests to Gus that he has two options: option "A" being to kill Walt and Jesse, and option "B" being to let it go and keep Walt cooking, meanwhile forgetting Jesse ever existed. It seems to me that Gus chooses option B, when he says "You'll need a new lab assistant [...] this time I choose".

However, even though the agreement seems to materialise with Gale back on the team, Walt admits to knowing about Gus's intentions (revealed to Gale a few scenes earlier) when visiting Laser Tag with Saul. He seems far beyond suspicious about it. How can he be so sure?

Answer 1

I think there are two main points that explain this:

• First and foremost, this whole matter involves drug producers (Walter, Gus) that have already killed in the past to protect their interests. Walter is no dupe, he knows killing those two gunmen in S3E12 "Half Measures" (don't forget those were Gus' men after all!) has irremediably wiped out any remaining goodwill Gus had towards him: Gus is a very cautious trafficker and Walter's actions are anathema to how he conducts his business. Remember than in the previous episode, Mike visited Walter at his house and hinted that the best course of action for him would be to get rid of Jesse (even if Mike's visit was not ordered by Gus): Walter knows how Gus and Mike think and that they won't hesitate to get rid of threats to the business.
• In addition, Walter notices that, after being enrolled back as his lab assistant, Gale is making many more questions about the cooking process than before, as if he was preparing to take over Walter's job. Excerpt from the laser tag conversation between Walter and Jesse:

  [Walter] How are you holding up?
  [Jesse groans] You?
  [Walter] I got my old job back. At least until they kill me and Gale takes over.
  [Jesse] He's their boy, huh?
  [Walter] He's their boy.
  [Jesse] How long you think you've got?
  [Walter] Well, he asks a lot of questions about the cooking process. I try to be as vague as possible, but I got that guy Victor watching me, listening to every word I say. Maybe the only thing that's saving me is Gale's fastidiousness. Once he feels confident that he knows my entire method...

Answer 2

Walt notices heightened interest from Gale about the meth production process and assumes that Gus has talked to Gale about taking over. Logically, he concludes Gus wants to get rid of him.

Answer 3

Walt is smart and knows that there's no way that Gus is going to let things slide. Part of the reason he asked to replace Gale in the first place was because Gale was a very astute student and Walt was worried about being replaceable. Now that Walt had deliberately defied Gus, it was only a matter of time before he would be taken out and Walt knew that having Gale back meant Gus was hoping he could come up to speed so that they would no longer need Walt.