In my opinion, there's no real way to defend/explain/rationalize Gus ominously stopping at just the right moment to go stare out in Walt's direction in the parking lot anyway other than in the words of Marlo Stanfield: That's some Spider-man shit there.
I think the writers just can't help themselves with Gus, he became a pulp/comic book-style supervillain this season, complete with a semi-tragic origin story. I can live with it, it's part of the show's DNA that it's a crazy crime comic story that likes to let its bad guys go a little out there now and again, and it does it well. Plus Esposito is completely nailing the character they've given him (what a painful reminder of how utterly they wasted him in the unfortunate final season of Homicide...if they'd given him anything to work with, there's no question Esposito gone have gone a long way towards filling the Andre Braugher void.)
I do, however, worry about the resolution of this Brock poisoning thing. Earlier in the season, before I came to embrace Gus as Dr. Doom/The Kingpin, I had some trouble buying into his plot to win over Jesse that involved assumptions that Jesse wouldn't murder the hell out of the two shotgun-toting guys assigned to scare him and take over forever with the money, leaving Gus to solely depend on Walt again. But this one is potentially even more convulted.
If Gus did it, his plot depends on him super secretly knowing about the killer cigarette all season long, on Jesse not noticing the cigarette missing for the few hours in which it had to get to Brock and instantly work on him, on Jesse pinning the blame on Walt and on an armed, rightly paranoid, murder-capable Walt not blowing a revenge-bound Jesse away or them killing each other, leaving Gus with no one to make his supermeth. That is a very complex, unlikely series of events that has to happen in pretty tiny window of time.
If Walt did it...I think the degree of difficulty for making this all happen in that amount of time possibly doubles (we have to assume that Huell is a Ricky Jay-level master of misdirection to swap cigarette packs during his brief frisking of Jesse, and the entire plan hinges on Jesse actually bothering to heed Saul's calls to come in that afternoon, plus Walt still has to get Saul or Huell to tell him who Brock is and then go poison him) and there are similar logic leaps in play (everything then depends on Jesse realizing what happened before Tyrus wanders by to happily kill Walt, Jesse actually going to Walt and not beating Walt to death or blowing him away), but this feels like it could be the kind of huge moment the season is building up to, as Walt shoots past the entire cast on the evil scumbag scale. It would also nicely set up a final 16 episode of Jesse working for new drug lord Heisenberg and Jesse then needing to find out that Walt was either directly or indirectly responsible for the deaths of Jane and Brock so he can decide to kill Walt.
So, yeah, I'm a little worried about them resolving the actual details of this latest plot point and it following on the kinda sloppy business with Ted Beneke's return, but I'm willing to hold off to see the reveal of what actually happened next week because of how wildly entertaining Season 3 and the back end of this season have been.