Note that there is only one plugin for SMS and GG.
From plugin readme:
Dowload plug in here.Although it's possible to use one DLL for both Sega Master System and Game Gear
files, I strongly recommend to copy the DLL and give it a different name for
either Sega Master System files or Game Gear files. For example, if you use
sms.dll for Sega Master System files, copy the file to gg.dll for Game Gear
files. If you want RomCenter to use the plug-in, make sure that the DAT file
contains a line that looks like this:
Use the gg and sms dat available at uCON64 homepage for best results.
Many thanks to dbjh and Dweezledap for their work.