You need two pieces of software: A sequencer (e.g. cubase, sonar, logic) and Gigastudio.
You're sequencer records your compositions as MIDI. On playback it routes the midi notes to GigaStudio where "sound samples" are triggered and played back in real time i.e. the sounds of musical instruments (or other noises).
You can then either use a) GigaStudio's on board effects and mixer to get the balance between the various instruments correct, or b) you can route GigaStudio's audio back into your sequencer to be recorded and mixed there, or c) a bit of both.
Think of GigaStudio as a soundmodule. You can play the sounds live by routing a midi keyboard directly to it, or via your sequencer.
Remember: You will also need to buy the appropariate sound samples libraries for GigaStudio to use, although the latest version does come with some demo samples libraries. It doesn't come with a vast pallete of samples included, and you will soon want to buy the sounds appropraite for your genre of music. Have a look on the Giga web site to see what freebies come withthe various versions.
And also remember, in terms of hardware you need a high spec PC (3Ghz) with two (ideally 3) hardrives, a GigaStudio compatible soundcard, and a midi keyboard.