Prove that the scalar curvature is twice the value of the Gaussian curvature

Question

I would like to show that the scalar curvature $R$ is twice the value of the Gaussian curvature $K$: $$R=2K.$$ I know that $$R=g^{ij}R_{ij}=Kg^{ij}g_{ij}.$$ But somehow I don't know what's going on in the summation convention. Why do we have $g^{ij}g_{ij}=2$ instead of $g^{ij}g_{ij}=1$? Thank you.