Gief doesn't beat Sagat. Sagat players get sloppy or Gief players get a lucky knockdown and start the body splash to lariat or SPD 50/50 guessing game.
But as a character, Gief is screwed in that match-up.
Agreed, only way Gief wins is two things...#1 The Sagat player is playing far too aggressively instead of patiently zoning and dominating Gief like that, or #2 Gief does get in and as you said, gets a knockdown or gets close.
Even then, Sagat can turn the tide on a whim with Ex-Tiger Uppercut, if your Gief and you play a Sagat whom is simply zoning with Low Tiger, and long pokes. Gief simply walks into a wall, because unlike other characters like Ryu, punishing Sagat's low Tigers are alot more difficult, due to the fact that Sagat's tiger shots are just simply faster, and more importantly the recovery of them, makes him far safer. You throw in Ex-Tiger shots too, and it makes it incredibly difficult, and again, even if Gief gets in, he still has to be weary because Sagat has the options and tools to knock Gief to the ground, and push him back at square one again.
Any character that can keep Zangief out of SPD effectively will always give him trouble, in fact, even if you go as far back as SFII, Sagat has always been one of those characters whom Gief had alot of trouble with.
In comparison, the match up isn't as bad as the old SFII days, due to the fact that Low Tigers do not travel at 10,000 mph, the damage off of Tiger Shots are nowhere near the high damage they once were. Stages were larger in SFII helping out projectile characters, and Gief has more tools in IV to get around projectiles in comparison, so the match up is easier, but it is still undeniable that Sagat holds the edge.
However, although Sagat has the advantage, if Zangief faced someone like Ryu, the match up is alot better (5-5), and well really depend upon who plays better, unlike the Sagat match up, to where if the Sagat player just plays defensive instead of the annoying "rushdown" mentality, Gief's in trouble.