Over the years I've owned a couple of GK (Gallien-Krueger) amps including bass amps and more recently (since mid-90's) a GK 200MV microphone monitor which I use for my keyboard rig. It's perfect for both gigs in small rooms and as a stage monitor that's easy to tie into a house sound system. GK's have great sound. Now I can tap into the GK technology in a whole new way with the iRiffPort and GK amp simulators from PocketLabworks.
The hardware of their system is the iRiffPort guitar/bass interface for iPhone, iPod Touch and iPad. With one cable, you plug your guitar directly into the device with the 30-pin dock connector. There's even a stereo headphone and line-level output giving you plenty of connectivity options. The cable is very well assembled and substantial in size. Not at all flimsy . . .
The PocketAmp app is designed for guitarists with a number of amp simulators/speaker cabinet options. Also included are echo and four other effects including chorus, flanger, rotary, and tremolo. Settings can be saved as presets allowing for quick access to your favorite rig. PocketGK is designed for bassists. There are two cabinet settings and EQ controls.
What really makes both apps stand out are their ability to play any track from your iTunes library from within the app. More importantly are the controls that let you change tempo and pitch independently as well as set loop points and control the balance of amp output versus track playback. This is an extremely useful practice tool.
All in all I think PocketLabworks got it right. Great sounding amp simulations, flexible design and practice tools.