Magnetic frustration is the phenomenon resulting from inability to satisfy all magnetic interactions present in a system. It has drawn lots of attention as it is the source of exotic behavior of many magnetic materials. Although it has been thoroughly studied in various groups of compounds, not much of an effort was put in its examination in half-garnet lattice family and especially terbium gallium garnet(Tb3Ga5O12). TGG is a promising candidate for a host of magnetically frustrated spin liquid regime. However, its existencein it has never been confirmed. Defining a crystal field scheme is a crucial, initial step in investigation of any rare earth based magnet and inelastic neutron scattering is the most reliable method of doing this. We propose a combined study at LET and MARI instruments aiming at observation of Tb3+ ions' full crystal field spectrum in terbium gallium garnet.