Gaara is impressive, don't get me wrong. I think he could take both the 3rd and 4th Raikage. But Onoki is a different matter. He just seems to be a tier above Gaara.
-Gaara can create a sand shield over the village (but at great effort). Onoki can casually carry a whole island across a vast distance.
-Gaara can simulate flying for himself and others. Onoki can actually make them fly
- Garaa can survive a meteor strike... Onoki can actually stop it (one at least).
-Gaara can crush his foes. Onoki atomises them.
And this is all sub-prime Onoki, who is assumedly weaker than prime Onoki.
Onoki has a variety of techniques too, from stone golems to rock clones, which will at least slow Gaara down, and seems to have large chakra reserves to spam them. Assumedly he could petrify or weigh down Gaara's sand if he managed to get a grip on him too. None of which accounts for his ace in the hole- Dust techniques. I'm not even sure Gaara can slow Dust attacks down, and when using his full strength (even without Tsunade), Onoki seems to have alot of reach and distance on these.
I'm just not seeing it. Onoki is an insanely powerful character, as is his teacher Mu (which only makes it more insane that EMS Madara apparently effortlessly schooled them both at the same time without even trying). Only the really high end guys have a chance against him (like the current Naruto, Sasuke, Nagato, Itachi, Minato, Hasmirama, etc).