The 'Bani' of Guru Gobind Singh Ji (really?) finds no place in Guru Granth Sahib Ji. By Guru Granth Sahib Ji I mean the 'Bir' that was bestowed upon the Guruship by Guru Gobind Singh Ji HIMSELF. There could be no reason why the 'Bani' was worthy of being included in the Guru Granth Sahib Ji ans was not included, because the Tenth Master has all the time, opportunity and authority to do so.
Seriously, Talking about the 'Dasam Granth', authentic researches have never pointed out that whole of the Dasam Bani could be the work of Guru Gobind Singh Ji. Not many of us would know that the SGPC itself had issued a letter (Letter no.36672 dated 3,August,1973) excluding the 'Charitropakhyan' (which forms a major portion of the Dasam) from the Dasam Granth adding that it was only the translation of mythological Hindu texts, not Dasam Bani !!