= Solution
Substitute the <Focusing Schrödinger lens ansatz> into
$$
i u_t+\Delta u+|u|^2u=0.
$$
The imaginary terms proportional to $P_\eta+y\cdot\nabla P_\eta$ cancel because $b=-\lambda\lambda_t$. After multiplying the remaining real equation by $\lambda^3$, one obtains
$$
\Delta P_\eta+P_\eta^3
-\lambda^2\gamma_tP_\eta
+\frac{\lambda^2b_t+b^2}{4}|y|^2P_\eta=0.
$$
The dynamical system gives $\lambda^2b_t+b^2=-\eta$. Comparison with the equation for $P_\eta$ therefore requires
$$
\gamma_\eta'(t)=\frac1{\lambda_\eta(t)^2}
=\frac1{t^2+\eta}.
$$
Thus, up to an arbitrary constant phase,
$$
\boxed{\gamma_\eta(t)
=\gamma_\eta(-1)
+\frac1{\sqrt\eta}
\left(\arctan\frac t{\sqrt\eta}
+\arctan\frac1{\sqrt\eta}\right)}.
$$
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