From Hindu Devotional Blog:
Vishwamitra Author of Gayatri Mantra
Vishwamitra Rishi is credited as the author of Gayatri Mantra – the highly revered mantra of Hindus. Gayatri Mantra is a hymn of the Rig Veda which consists of 14 syllables, each of which holds an important meaning. Sage Vishvamitra has authored most of Mandala 3 of the Rigveda which include 62 hymns dedicated to Lord Agni and Lord Indra.