On a keyboard, a G played with a Gb next to the octave G would sound ok, but if played next to each other will sound pretty discordant. Does BIAB know that? Perhaps a GMaj7 with the b5 played as a slash Bass type of chord would sound a bit better.
Music theory is not my strong suit (in fact it is my weakest I would have to admit) but I would like to hear what those in the know have to say about this.