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.
Günter Guillaume died in 1995.
Guillaume Soisson died in 1938.
Guillaume Schnaebelé died in 1900.
Guillaume Guiart died in 1316.
Guillaume Crétin died in 1525.
Guillaume Taraval died in 1750.
Guillaume Tronchet died in 1959.
Guillaume Tirel died in 1395.
Guillaume Arrufat died in 1311.
Guillaume d'Ercuis died in 1315.
Gustave Guillaume died in 1960.