Gündüz Alp | Son of Ertuğrul | Elder Brother of Osman Gazi

 Gündüz Alp was the son of Ertuğrul  (13th century) and Elder Brother of Osman I, the founder of the Ottoman Dynasty. According to some sources, the name of one of the sons of Ertuğrul was also Gündüz Alp and thus the brother of Osman I. Ottoman histories, written around the 15th century, differ in details about Osman I's ancestry.

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. Many contemporary history professors like İlber Ortaylı, Halil İnalcık, Erhan Afyoncu, Yılmaz Öztuna, and Osman Turan 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.

According to Hasan bin Mahmûd el-Bayâtî's Câm-ı Cem-Âyîn,  Âşıkpaşazâde's History of Âşıkpaşazâde and Neşrî's Kitâb-ı Cihannümâ  the name of one of the sons of Ertuğrul and one of the brothers of Osman I was Gündüz, who had a son named Aydoğdu Bey. Some other sources also indicate that he had another son, Aktimur Bey, who was a soldier and government official in the establishment of the Ottoman Empire. This Gündüz had another brother named Saru-Yatı (Savcı Beg). Osman I may also have had a nephew named Gündüz.

According to 15th century Ottoman writers Enveri and Karamani Mehmet Pasha,Gündüz Alp was the father of Ertuğrul. Yazıcızâde Âli' Tevârih-i Al-i Selçuk (15th century) indicated Gök Alp, the brother of Gündüz Alp as the father of Ertuğrul. Some other writers in their published works like 15th century historians; Şükrullah's Behcetü't Tevârîh  Hasan bin Mahmûd el-Bayâtî's Câm-ı Cem-Âyîn, Âşıkpaşazâde's History of Âşıkpaşazâde, Neşri's Kitâb-ı Cihannümâ and early 16th century Oruç Bey's Tevârîh-i Âl-i Osman  had asserted that Süleyman Şah was the father of Ertuğrul. Halil İnalcık argues that the latter have confused Süleyman Şah with the grandfather of Gündüz Alp, namely Mir Süleymân Alp. Three coins which read "Osman bin Ertuğrul bin Gündüz Alp" supports the view that Gündüz was Ertuğrul's father.