Gündüz Alp was the likely father of Ertuğrul (13th century) and grandfather of Osman I, the founder of the Ottoman Dynasty.

[1] According to 15th century Ottoman writers Enveri[7] and Karamani Mehmet Pasha,[5] Gündüz Alp was the father of Ertuğrul.

[14] Family tree in Neşrî's Kitâb-ı Cihannümâ:[3] Three coins minted by Osman I in Yenişehir-Bursa during his reign which reads "Osman bin Ertuğrul bin Gündüz Alp" showed that the name of the father of Ertuğrul was Gündüz.

[11][12][13] Many contemporary history professors like İlber Ortaylı, Halil İnalcık, Erhan Afyoncu,[14] Yılmaz Öztuna[16] and Osman Turan [tr][17] demonstrated that those history books written more than six hundred years ago had erred in determining the father of Ertuğrul for some reasons, and had given Ertuğrul's father name incorrectly as Suleyman Shah instead of Gündüz Alp because of the similarity of the name of Kutalmışoğlu Suleyman Shah the founder of Seljuks of Rum, and Osman I's presumed ancestor Suleyman Shah.

[19] Some other sources also indicate that he had another son, Aktimur Bey[20] who was a soldier and government official in the establishment of the Ottoman Empire.