In my opinion it is better to work in an explicit covariant form. In my answer I will use two different definitions, the Greek indexes always run from $0$ to $3$ and Latin indexes from $1$ to $3$ and the metric $g_{\mu\nu}$ has signature $(-1,1,1,1)$.
To translate the expressions to a explicit covariant form we define some timelike vector field $v^\mu$. We can define an adapted coordinate system such that $v^\mu = \delta^\mu{}0$ and, therefore, $$g{00} = g_{\mu\nu}v^\mu{}v^\nu.$$ However, it is better (in my opinion) to not work in such coordinate system, in this case we always use expression like the right hand side of the above, i.e., $v^2 \equiv g_{\mu\nu}v^\mu{}v^\nu$.
The projected metric from your equation can be expressed as (where I reworked the signs to conform to my definitions) $$\gamma_{ij} = g_{ij} - \frac{g_{0i}g_{0j}}{g_{00}} = g_{ij} - \frac{v^\mu v^\nu g_{\mu i}g_{\nu j}}{v^2} = g_{ij} + \frac{v_i v_j}{\sqrt{-v^2}\sqrt{-v^2}} = g_{ij} + n_i n_j,$$ where we defined, naturally, $v_\mu = g_{\mu\nu}v^\nu$ and the normalized version of $v^\mu$, i.e., $n^\mu \equiv v^\mu/\sqrt{-v^2}$. Note that here we introduced the rest of the coordinate basis given by the vector fields $e_i{}^\mu$ such that $g_{ij} = e_i{}^\mu{}e_j{}^\nu{}g_{\mu\nu}$, $v_i = v_\mu{}e_i{}^\mu$, etc. 818
Then, to avoid the introduction of coordinates we define the projector $$\gamma_{\mu\nu} = g_{\mu\nu} + n_\mu{}n_\nu,$$ which reduces to the spatial metric in the adapted coordinate system and in general works as a projector $\gamma_{\mu\nu}n^\nu = 0$.
One can introduce the determinant in a covariant format using the space of totally antisymmetric tensors of type $(4,0)$, e.g. $\epsilon^{\mu\nu\alpha\beta}$ which is antisymmetric in any two adjacent indexes. We define $\epsilon^{\mu\nu\alpha\beta}$ using the expression $$\epsilon^{\mu\nu\alpha\beta}\epsilon_{\mu\nu\alpha\beta} = -4!,$$ where the indexes were lowered using the metric $g_{\mu\nu}$, this defines $\epsilon^{\mu\nu\alpha\beta}$ up to a signal since the space of totally antisymmetry tensors is one dimensional. Using the definition of determinant in terms of the Levi-Civita symbol it is easy to show (you can take a look at Appendix B of Wald 1984) that $$v^\mu{}e_1{}^\nu{}e_2{}^\alpha{}e_3{}^\beta\epsilon_{\mu\nu\alpha\beta} = \epsilon_{0123} = \sqrt{-g},$$ where $g$ is the determinant of $g_{\mu\nu}$ calculated in the coordinate basis formed by $(v^\mu,e_i{}^\mu)$. 578