okay, but who can loudly claim with full faith that "Guru Gobing Singh did not write dasam granth"... and if someone can.... then ask we should ask... are you now at the level of Guru Gobind Singh, or even high to dictate what they wrote and didn't.
How could he write it if he was dead before it ever appeared on the scene?
Dasam Pita died in 1708. Sri Dasam Granth Sahib is published in 1901/1903 based on a disputed collection of pothis.
If you are talking about individual hymns, such as Jaap Sahib, then you have an excellent question. In 2007 SGPC appointed a commission to investigate just such questions using modern authentication methods. So far nothing has come of it.