yesterday I posted that the HDMI Sound problem with the mastersystem roms can be fixed by using the Genesis-Plus-GX emulator. Today I installed a new RetroPie and there’s no Genesis-Plus-GX-core installed anymore.
You are right – for some reasons the Genesis GX core was discarded with the introduction of Osmose. I have updated the RetroPie script just now, and Genesis GX can be selected from the source-based/custom menu again! The updated binaries are uploading right now.