B.I.: I also have Cakewalk's Gtr Trks Pro, and have recorded 5 complete CD projects, both solo and for the Bluegrass band I play in. For proper recording of acoustic instruments into your computer, you will need some sort of interface between a microphone and the computer.

I use a small, cheap Behringer board (there about $49 in Musiciansfriend), plug a mic into it, and route the board output into the microphone jack of my computer. You will need various adapters for this, easily obtained at Radio Shack.

Once this connection is made, you can adjust the board level to get the correct recording level in the GTPro.

If you are going to do a lot of recording, this setup, or one similar is a good low cost way to do it. You can then also plug in other things directly to the board: electric guitar, bass guitar, cassette/CD players, etc.