GB has chords in it. The basic version has many and you can add more with additional apple loops. Look on You Tube there a hundreds of tutorials Good Luck

From BIAB save it to midi then drag it into GB. All of your individual instrument tracks will be created. From there you can do what you want. To change chords in GB you need to change the pitch. Double click on the instrument track to bring up the window on the bottom. Then use the pitch slider to go up or down. This changes the chords. If you are in C then +1 = C#, +5 = F, +7=G etc. work your way up to the chord you need it to change to. Hope this helps.