Guillaume Costeley died in 1606.
Probably the reason you couldn't find it is that the last name is spelled 'Costeley'. The original poem is by Pierre de Ronsard. https://vault2.secured-url.com/handlo/pdf/a-f/CAFM--As.pdf How I found it: Google search #1 - search for Guillaume Costelet. Google corrected the spelling. Google search #2 - search for Guillaume Costeley Cassandre.
Guillaume Duprat died in 1560.
Guillaume Durand died in 1296.
Guillaume Colletet died in 1659.
Guillaume Couture died in 1701.
Guillaume Voiriot died in 1799.
Guillaume Courtet died in 1637.
Albert Guillaume died in 1942.
Guillaume Chasteau died in 1683.
Guillaume Courtois died in 1679.
Guillaume Lekeu died in 1894.
Guillaume Morel died in 1564.