We consider the flows generated by generic gradients of Morse maps of a closed connected manifold $M$ to a circle. To each such flow we associate an invariant counting the closed orbits of the flow. Each closed orbit is counted with the weight derived from its index and homotopy class. The resulting invariant is called the eta function, and lies in a suitable quotient of the Novikov completion of the group ring of the fundamental group of $M$. Its abelianization coincides with the logarithm of the twisted Lefschetz zeta function of the flow. For $C^0$-generic gradients we obtain a formula expressing the eta function in terms of the torsion of a special homotopy equivalence between the Novikov complex of the gradient flow and the completed simplicial chain complex of the universal cover of $M$.