The official Gunpey site has posted video (last button on the left) of the previously-announced in-game sequencer. NeoGAF's A_Lee_N both posted about the video and provided some stills of the feature.
The sequencer seems to be a simple five-channel type and appears to provide the ability to make your own music to play in the game. Namco Bandai has announced they will bring Gunpey stateside this holiday season.