[en] β-delayed one-neutron and two-neutron branching ratios (P_1n and P_2n) have been measured in the decay of A=84 to 87 Ga isotopes at the Radioactive-Isotope Beam Factory (RIBF) at the RIKEN Nishina Center using a high-efficiency array of 3He neutron counters (BRIKEN). Two-neutron emission was observed in the decay of ^84,85,87Ga for the first time and the branching ratios were measured to be P_2n=1.6(2)%,1.3(2)%, and 10.2(28)_stat(5)_sys%, respectively. One-neutron branching ratio of ⁸⁷Ga(P_1n=81(9)_stat(8)_sys%) and half-life of 29(4) ms were measured for the first time. The branching ratios of ⁸⁶Ga were also measured to be P_1n=74(2)_stat(8)_sys% and 16.2(9)_stat(6)_sys% with better precision than a previous study. The observation that P_1n>P_2n for both ^86,87Ga was unexpected and is interpreted as a signature of dominating one-neutron emission from the two-neutron unbound excited states in ^86,87Ga. In order to interpret the experimental results, shell-model and Hauser-Feshbach statistical model calculations of delayed particle and γ-ray emission probabilities were performed. This model framework reproduces the experimental results. The shell model alone predicts P_2n significantly larger than P_1n for the ⁸⁷Ga decay, and it is necessary to invoke a statistical description to successfully explain the observation that P_1n > P_2n. Our new results demonstrate the relevance and importance of a statistical description of neutron emission for the prediction of the decay properties of multineutron emitters and that it must be included in the r-process modeling.