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 Podrovnik goes by Guillaume Podrovnik.

Guillaume Podrovnik's birth name is Guillaume Wojazer.

Guillaume Hanoun's birth name is Guillaume Hanoun.

Guillaume Catala's birth name is Guillaume Catala Philippe.

Guillaume Vogels has written:

'Guillaume Vogels, 1836-1896'

Guillaume Radot's birth name is Guillaume Simon Charles Radot.

Guillaume Debroise's birth name is Guillaume Marc Frederic Debroise.

Guillaume Filleul's birth name is Joseph Paul Guillaume Filleul.

Guillaume Le Bras goes by Guillaume Le Braz.

Guillaume de Palerne. has written:

'Guillaume de Palerne'

Guillaume Philippe Benoist died in 1770.

Guillaume Depardieu's birth name is Guillaume Jean Maxime Antoine Depardieu.

Philippe de Vitry and Guillaume de Machaut

Guillaume was born on November 11, 1981.

Guillaume was born on November 11, 1981.

Guillaume Bilodeau is 6' 1".

Guillaume Rousselet is 185 cm.

Guillaume Tauveron is 178 cm.

Guillaume Dabinpons is 187 cm.

Guillaume Dauphinais is 6' 2".

Guillaume Debroise is 179 cm.

Guillaume Depardieu is 6' 1".

Guillaume Moiton is 6' 2".

Guillaume Latendresse is 6' 2".

Guillaume Durand is 183 cm.

Guillaume Dustan was born in 1965.

Eddie Guillaume is 5' 11".

Robert Guillaume is 5' 10".

Guillaume Hissung is 176 cm.

Guillaume Adam was born in 1973.

Guillaume Catala is 178 cm.

Guillaume Canet is 180 cm.

Guillaume Bousses is 184 cm.

Guillaume Clement is 175 cm.

Günter Guillaume died in 1995.

Günter Guillaume was born in 1927.

Guillaume Soisson died in 1938.

Guillaume Soisson was born in 1866.

Guillaume Schnaebelé was born in 1831.

Guillaume Schnaebelé died in 1900.

The population of Puy-Guillaume is 2,624.

Guillaume Guiart died in 1316.

Guillaume Crétin was born in 1460.

Guillaume Crétin died in 1525.

Guillaume Taraval died in 1750.

Guillaume Taraval was born in 1701.

Guillaume Tronchet was born in 1867.

Guillaume Tronchet died in 1959.