Gelbart's early interest in science was inspired by his time as an undergraduate researcher in the molecular spectroscopy group of William Klemperer at Harvard.

Rice, Karl Freed, and Joshua Jortner, he developed the modern theory of non-radiative processes ("radiationless transitions") in molecular photophysics.

Shortly after moving there he began a 40-year collaboration with Avinoam Ben-Shaul on statistical-thermodynamic models of liquid crystal systems, polymer and polyelectrolyte (in particular, DNA) solutions, and colloidal suspensions, and on the self-assembly theory of micelles, surfactant monolayers, and biological membranes.

Early results included: the first measurement of pressure inside DNA viruses, establishing that it is as high as tens of atmospheres depending on genome length and ambient salt concentrations;[9] and the demonstration that capsid proteins from certain viruses are capable of complete in vitro packaging of a broad range of lengths of heterologous RNA.

[11] In 1987 Gelbart was elected a Fellow of the American Physical Society "for his many contributions to the light scattering and phase transition properties of simple fluids, liquid crystals, and surfactant solutions".