Our contribution highlights for the first time the French cleric João Pedro Gay (1815-1891) as a leading figure of Guaraní linguistic research in the nineteenth century. We are basing our analysis on a poem in Guarani copied by his hand from the local newspaper Eco de Corrientes in 1867. This poem is in itself a still unknown valuable source for XIX century Correntinean Guarani and represents a part of a larger legacy of Gay as researcher of the Guarani language. We contextualize the poem in its historical and sociolinguistic framework taking into account the specific situation in Argentine during the War of The Triple Alliance and its underlying linguistic ideologies. We conclude defining Gay’s copy of the poem as part of an important linguistic contribution enabling us to acquire an outstanding and comprehensive knowledge about the history of Guarani varieties in this region.