- you can save the wav or aac file of the song in iReal Pro (set your repeats, key, tempo, style, mixer volumes etc. before doing this) then drag into an audio track in GB
- or iReal Pro can play in the background, so I presume connecting your guitar into GB and playing it live with its effects should still work (although I have not tested this)
- the audio output menu is bottom left if needed (external speakers or audio interface)

There is no specific tutorial on this. Does this give you enough information, Len?