Here's the Milestone 6 release of my version of Gens, Gens/GS. Milestone 6 introduces the new MDP plugin system. Currently, plugins must be built into the Gens executable, but the next version will add support for external plugins.
Also, Gens/GS m6 has several new renderers: Scale3x, Scale4x, hq3x, and hq4x.
For more information, see http://info.sonicretro.org/Gens/GS