Ok, I see. But still... if w > g survived, then g itself could have survived too. Are there any examples Germanic words with g being borrowed into French with j?
I'm guessing this is related to the oddities in the second paragraph of gain#Etymology 3? If French gagner > older gaaignier > Gothic gaaiginōn (gaaiginōn), then any /w/ > /g/ mutation seems irrelevant. And what of Spanish ganar?