After Gus fired Walt, prefigured he would kill Hank, threatened Walt's family and left him in the desert near the end of Breaking Bad S4E11 »Crawl Space«, it doesn't seem too hard for Gus to anticipate that Walt might try to kill him. Of course Gus is surrounded by bodyguards all the time, but as a very cautious man, wouldn't it have made sense for him to additionally have somebody shadow Walt in order to avoid unwanted »surprises«?
This of course only applies for the time between the mentioned scene in the desert and the time Gus finds out that Hank is under DEA protection, from when Gus wants to kill Walt anyway. But by then, Walt has already disappeared from his radar and his family is safe at Hank and Marie. But if Gus had kept an eye on Walt from right after he left him at the desert, he'd have probably never lost his track at all.
And probably Gus' face would still be in it's original place.