According to the Internet source given below, Guillaume Costeley was a French composer. By 1560 he was court organist to Charles IX of France, and in 1575 founded the Évreux 'Puy de musique', an annual prize for composition, in honour of St Cecilia. He wrote over 100 chansons, which continued to be reprinted until 1633, and was the leading chanson composer of the late sixteenth century. He belonged for a time to the poet Baïf's circle.
||Brief biography by www.hoasm.org.