According to wikipedia:
Guru Arjan, the fifth Sikh Guru, added religious mystics belonging to other religions into the holy scriptures that would eventually become the Guru Granth Sahib.
Were any Islamic mystics included within them?
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There are 6 writers with Muslim background, whose hymns are incorporated in Guru Granth Sahib, whom Sikhs call Bhagats(Devotee):
1) Devotee Kabir
2) Devotee Sheikh Fareed
3) Devotee Sadhana Qasai
4) Devotee Satta Dum
5) Devotee Balwand Rai
6) Devotee Bhikhan