If electric field at a point would vary as r^(-2.5) then would gauss law still be valid?

no gauss law will not be valid because gauss law is valid only when electric field varies as r^(-2)

Approved

Why is it so?

For this you should go through the deduction of Gauss`s Law from Coulomb`s law. In Coulomb`s law the electric field itself varies as 1/r^2. Had that not been the case then we would have not come down to the result of Gauss`s Law.