The author describes he is overriding the V-Sync parameter in Direct3D and OpenGL (and apparently doing some additional processing with DXVA).
Whatever it is, it could never replace G-Sync. There is a big design difference between V-Sync and G-Sync. V-Sync is a software solution that synchronizes rendering with the monitor refresh rate. Say you have a 60Hz monitor, it will then cap the framerate to 60 FPS and enforce that by basically idling the GPU (and often the CPU too) in the remaining time. G-Sync is a hardware solution that does the exact opposite, it synchronizes the monitor refresh rate with the GPU output (as far as the maximum the G-Sync monitor can handle). I cannot give an opinion on how much of a visual effect that has, as I never sat in front of a G-Sync monitor, but it's important to distinguish the two.
Reading further it seems he's setting up an extra framebuffer (or more, if I read right) independently of driver-based double/triple buffering. In theory this could smooth out some frame drops or 'stuttering', but also is likely to introduce input lag. However, reading comments on the most recent posting some chap is saying his input lag reduced...