Yes. Guga does not mean 'salted gannet'. It means 'young gannet'. They are not salted when you catch them though they are by the time you eat them. The word is not usually translated. I understand it is an 'acquired' taste. And they right pen and ink when they are being cooked.
Yes there is some overlap between glè and really but this is because really has a range of meanings. Certainly it would be a valid translation, but I am not sure it is the best translation and it is certainly not the word that gives the best impression of what glè means. I would stick with very.
Salt is sometimes used as a synonym for salted, so it would be understood the same way - although I have never heard it. But this is mainly because no one talks about salt(ed) gannet at all. Several people have argued unopposed on various questions that it is simply a wrong translation. Most people say guga in English. Technically it means a 'young gannet', whether salted or still alive.