I guess it was just overlooked? This adds a separate list of games for that system. It's very similar to the code for the GBC. Paste it in after Gamecube:
<system>
<fullname>Game Boy</fullname>
<name>gb</name>
<path>[CHANGE THIS TO YOUR ROMS DIRECTORY]</path>
<extension>.gb .zip .ZIP .7z</extension>
<command>C:\Users\[YOUR USERNAME]\.emulationstation\systems\retroarch\retroarch.exe -L C:\Users\[YOUR USERNAME]\.emulationstation\systems\retroarch\cores\gambatte_libretro.dll %ROM%</command>
<platform>gb</platform>
<theme>gb</theme>
</system>
I guess it was just overlooked? This adds a separate list of games for that system. It's very similar to the code for the GBC. Paste it in after Gamecube: