Yesterday I ran in to matttt's excellent video exploring this question, i.e. "was Ballad of the Salt Sea (1967) the first GN?," altho to be clear-- it's mostly about the total comics career of legendary Italian artist / writer Hugo Pratt.
TBH, I wasn't really expecting to sit through a 17min video, but "matttt" is just such a great talent, that it wasn't hard. I only wish he covered more Euro comics specifically, but in any case, he's a terrrrific, home-grown comics historian who speaks with passion and precision. (did you like that?)
NOTES: 1) there's a built-in sponsor-ad starting just after 5min that lasts a full minute, 2) If you want to skip specifically to the Corto Maltese stuff, you can start about halfway through, in which matttt gets in to what makes the series so special:
-----> https://www.youtube.com/watch?v=jlIKSeNJrC8 <-----
As for what really was the first GN? Good question. There's an old idea that a bunch of short comics collected in to hardcover format fit this definition, but I'm not sure I buy that. Personally, I tend to think of a 'proper GN' as a long-form narrative that's not just gag-related. So the Tintin albums would fit the bill for me, but in fact there are examples going back to the 1920's, possibly even earlier.
I guess matttt's proposal of Ballad of the Salt Sea as the first GN (in which Corto was only one of several prominent characters) is based on the idea that it was the first (modern-era at least) example of comics in fusion with literature, for what that's worth.
Now, I don't think we need to make war over this stuff; moreso, I find it a fun debate-topic in which to better understand comics as a whole.
Fair?
I really like Pratt's watercolors on the whole. He could go from 'primitive' all the way to 'tight & detailed,' depending on his mood and what he was working on.
https://www.google.com/search?q=%22hugo+pratt%22+%22watercolors%22&udm=2