What are the cores you would like to use?
From what I know, there would be 'cap32_libretro.dll' or 'crocods_libretro.dll'. Is that correct? Is there any other?
Otherwise, here is how to proceed:
1. Open "mGalaxy v8.0\Data\Definition\Emulators.xml" with a text editor
2. Do a search on "RetroArch", you should see something like that
3. Copy/Paste those 2 lines
<System name="Amstrad CPC" option="cap32_libretro.dll" script="false" cmd="-f -L cores\cap32_libretro.dll "%path\%file%ext"" extensions=".dsk,.zip"/>
<System name="Amstrad CPC" option="crocods_libretro.dll" script="false" cmd="-f -L cores\crocods_libretro.dll "%path\%file%ext"" extensions=".dsk,.zip"/>
Just like this
4.Save. From now you'll be able to choose Retroarch with Amstradt CPC
(This will be added to the next version of mGalaxy)