The "gallon" weapon reference could be either the .96 gal or the .52 gal, but I don't know why someone would recommend them to a charger player. My Splatoon instincts want to say she was suggesting one of the Splatling weapons. Perhaps she was suggesting the .96 gal Deco since that comes with Kraken Royale, a really good special for the Tower Control mode they were playing on, and its long range would be familiar to a charger player, but it could also be the regular .52 gal since the Killer Wail would let it pressure enemies off of the tower as well as disrupt their supposed area control.
I can't say for certain since I don't know the Japanese names for the weapons. From a cursory search, the Japanese refer to the .96 gal as .96ガロンデコ