Never said Gaara figures out how the jutsu works. If Deidara uses C4 like he used it
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, it's literally common sense as an aerial fighter to fly upward to avoid it. That shouldn't be controversial argument. Gaara knows Deidara uses bombs, so he should safely assume it's a bomb. Regardless if he thinks it's a dud or not, his initial reaction should be to fly farthest away possible from the bomb before it even goes off. Especially given it's size.
I know, which is why I specifically said ground
ed, not ground based. Basically saying that it can't fly on it's own. Deidara can send the C4 doll towards Gaara while it's mounted on a clay bird, however Gaara should stop it's efforts before it even gets remotely near him via
Suna Arare, which knocks Deidara and Onoki out the sky. Attempting the scatter the nano C4 bombs everywhere via another explosion is not a good tactic, considering Deidara can't track and control where all the bombs are going. They may very well scatter in his and/or Onoki's direction as well.
Tobi seemed to know all about it, and he wasn't partners with Deiadra as long as Sasori was.