General MIDI 2 is just an interface specification; software and/or hardware that conform to this specification are guaranteed to work with each other.
Palmidi's description "128 instruments" implies that it supports only the older General MIDI (1) specification.
A quick search on the Play Store does not turn up any app that mentions General MIDI 2. However, there might be some sequencer app that allows to add custom instruments.
Midi Clef can load SF2 (SoundFont) files containing custom instruments, so to be able to play back GM2 sounds, you need to get a GM2 soundfont.