derivative of $[n \times \nabla \eta]^2$ with respect to $\eta$

Question

I trying to implement an equation numerically, Numerically I can do it in, but was wondering if there is an analytical solution to it.

Here is the equation: $ f = [n \times \nabla \eta]^2$

Where, $n$ is a vector and $\eta$ is a scalar.

I need to implement $\frac{\partial f}{\partial \eta}$

Thanks in advance!