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 Wittouck died in 1829.
Guillaume Couture died in 1701.
Guillaume Morel died in 1564.
Guillaume Courtet died in 1637.
Gustave Guillaume died in 1960.
Guillaume Duprat died in 1560.
Guillaume Durand died in 1296.
Guillaume Colletet died in 1659.
Günter Guillaume died in 1995.
Guillaume Postel died in 1581.
Guillaume Adam died in 1329.