  Let $\mu_t$ denote the critical derivative Gibbs measure of branching
Brownian motion at time $t$. It has been proved by Madaule [Stochastic Process.
Appl., 126(2):470--502, 2016] and Maillard and Zeitouni [Ann. Inst. Henri
Poincar\'e Probab. Stat., 52(3):1144--1160, 2016] that $\mu_t$ converges weakly
to the random measure $Z_\infty \sqrt{2/\pi} x^2 e^{-x^2/2} \mathbb{1}_{x >0}
\,\mathrm{d} x$, where $Z_\infty$ is the limit of the derivative martingale. In
this paper, we are interested in the fluctuations that occur in this
convergence and prove for a large class of functions $F$ that \[ \sqrt{t}
\left( \int_{\mathbb{R}} F \,\mathrm{d} \mu_t - Z_\infty \int_0^\infty F(x)
\sqrt{\frac{2}{\pi}} x^2 e^{-x^2/2} \,\mathrm{d} x - \frac{c(F) \log
t}{\sqrt{t}} Z_\infty \right) \xrightarrow[t\to\infty]{} S^F_{Z_\infty}, \quad
\text{in law}, \] where $c(F)$ is a constant depending on $F$ and
$(S^F_r)_{r\geq0}$ is a 1-stable L\'evy process independent of $Z_\infty$.
Moreover, we extend this result to a functional convergence, and we identify
precisely the particles responsible for the fluctuations. In particular, this
proves the following result for the critical additive martingale $(W_t)_{t\geq
0}$: \[ \sqrt{t} \left( \sqrt{t} W_t - \sqrt{\frac{2}{\pi}} Z_\infty \right)
\xrightarrow[t\to\infty]{} S_{Z_\infty}, \quad \text{in law}, \] where here
$(S_r)_{r\geq0}$ is a Cauchy process independent of $Z_\infty$, confirming a
conjecture by Mueller and Munier [Phys. Rev. E, 90:042143, 2014] in the physics
literature.
