The NEWS-G experiment reports the first results from the search for low-mass WIMPs with SEDINE, a 60cm diameter Spherical Proportional Counter (SPC) prototype operated at the Modane Underground Laboratory (LSM). Competitive constraints were obtained on the spin-independent WIMP-nucleon interaction cross section in the GeV mass range and the most stringent limit ever reported worldwide was set for WIMP masses below 0.6 GeV. Not only do these results confirm the high potential of the novel detection technique of SPCs for light dark matter searches, they are extremely promising for the next phase of the experiment at SNOLAB.