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 Pellicier died in 1568.
Gustave Guillaume died in 1960.
Guillaume Duprat died in 1560.
Guillaume Durand died in 1296.
Guillaume Colletet died in 1659.
Guillaume Cale died in 1358.
Günter Guillaume died in 1995.
Guillaume Adam died in 1329.
Guillaume Postel died in 1581.
Guillaume Bautru died in 1665.
Augustin Guillaume died in 1983.