I never said Gaara was a Earth nature.We all know Gaara has wind chakra nature like naruto, but Sand is made from the earth, thus making it Doton.As in Gaara's sand is "Doton",so it's weak against all the other Kages.As for the Edo kages, Gaara father fought in the same manner as him , meaning Gaara knew his father battle tactics, which allow him to raise his sand above the Gold dust.Furthermore, they were fighting in a desert giving Gaara the ability to produce more sand.The same for the Mizukage, Gaara only defeated the Second Mizukage because he mixed in his father's Gold Dust with his sand.Not to mention neither Fourth Kages nor the Second Mizukage were fighting with their own will, but just reacting to enemies jutsus.Those advantages Gaara had in the desert fighting the Edo Kages are advantages he wouldn't have in a fight with any of the current Kages.