The PSF format was publicly documented by Neill Corlett in 2003, who also wrote a Winamp plug-in named "Highly Experimental" that plays PSF1 and PSF2 files.
This takes far less space than an equivalent streamed format of the same music (WAV, MP3) while still sounding high fidelity.
Several PSF sub-formats also have a miniPSF/PSFlib capability, wherein data used by multiple tracks is stored only once in an accompanying PSFlib file.
Further differences are stored in a miniPSF file, which can be compressed via zlib to further increase storage efficiency.
GSF players emulate the files as sound-only Game Boy Advance ROMs, and as such can be processor intensive when compared to mainstream audio formats.