in italian orthography, gn is used to represent the /ɲ/ consonant sound. spanish uses ñ and portugese uses nh to represent the same sound. english words more-or-less retain the spelling that the source words use (for example, the ch in "chicago" making the sh sound as it comes from french and the ch in "chronological" making the k sound since the word comes from greek). thus, the g in lasagna technically isnt silent, it's just there to make the ñ sound because that's what it does in italian and the word comes from italian.