Garg is best known for formulating the Leggett–Garg inequality, named for Anthony James Leggett and himself, which is a mathematical inequality fulfilled by all macrorealistic physical theories. He is also known for the Garg-Onuchic-Ambegaokar model of charge transfer. In addition, he discovered the phenomenon of topological quenching of the tunnel splitting in a toy Hamiltonian for spin tunneling, that was subsequently found experimentally in the magnetic molecule Fe8. His current research interests center around coherent state path integrals, especially as they pertain to quantum and semi-classical phenomena associated with the orientation of quantum mechanical spin.