In this work, the detonation wave structure and detonation parameters of mixtures of gelled nitromethane (NM) with hollow glass microballoons (GMBs) are investigated with a laser velocimetry technique. It is shown that the addition from 0.5 to 13 wt. % GMB with the mean diameter of 70 μm does not qualitatively change the reaction zone structure of gelled NM, and it corresponds to the Zeldovich–von Neumann–Döring theory, predicting the formation of the von Neumann spike. The detonation parameters of the explosive mixture monotonically decrease—in particular, the ideal detonation velocity drops from 6.3 km/s for gelled NM to 3.7 km/s for its mixture with 13 wt. % GMB. The addition of GMB leads to a significant reduction in the critical detonation diameter, which decreases from 16.5 mm in gelled NM to 3.5 mm in the mixture with 8 wt. % GMB. The influence of polymethylmethacrylate concentration in the mixture of gelled NM with GMB on the value of critical diameter of the mixture is observed.