The design and construction of compact and lightweight gantries for hadron therapy are essential steps toward wider accessibility to this cancer treatment. The use of a high-field steady-state toroidal gantry, i.e., GaToroid, represents an attractive alternative to the state-of-the-art with the potential of significantly reducing size and weight of present installations. It is interesting to conceive the use of High-Temperature Superconductors (HTS) conductors to reach magnetic fields beyond 8 T at relatively high temperatures, decreasing the beam bending radius and, at the same time, avoiding complex cryogenics systems in the hospitals. The construction of such a machine requires several steps of prototyping and this manuscript presents the design of the first GaToroid single-coil demonstrator, wound with ReBCO conductors. The demonstrator is a scaled version of a full-scale GaToroid coil, with the conductors spaced into four planar windings, i.e., grades, and wound as a double pancake. Two operating regimes are foreseen to test the demonstrator at different temperatures, currents and magnetic fields. The cable geometry, composed of four ReBCO non-twisted tapes, was validated through 1D quench propagation studies in both regimes, and the hot spot temperature is well below 100 K. Furthermore, three-dimensional mechanical simulations allowed to estimate the stress state on the conductor, as well as to define the impregnation strategy of the demonstrator. The answers provided by the manufacturing, powering and test of the proposed demonstrator can provide valuable insights for the future construction of full-scale GaToroid coils.