just fyi, Orochimaru was the one to kill the last Kazekage (without any damage to himself) and the 3rd was known as the strongest ninja ever

the title of Kage is very little indication of how strong someone is, and not that they're the strongest in the village either (at least in terms of the Leaf as Oro was considered to be the strongest but Yodaime was picked)
that being said, yes Kages are strong but they are not unbeatable (back to Kazekage being killed easily by Orochimaru without raising any suspicion)

i wouldnt compare Gaara to the 3rd