If he somehow does it he should be crowned best ever, same amount of grand slams and far better head to head.
I think that the distribution of Nadal's grand slams is a bit bad IMO. He has won 9/14 (64%) GS in French Open. For comparison, Fed has won 7/18 (39%) on Wimbledon.
Essentially, Rafa is the best ever tennis player in a Grand Slam, but take him out of French Open, and he has 'only' 5 GS. Take Fed out of Wimbledon, and he still has 11 GS, which by itself would put him in a shortlist for GOAT.
Then of course, Fed has more finals, more semis, more quarters, more weeks as No.1, more consecutive weeks as No.1 than any other tennis player in the history.
...
Even the record which is absolutely in Rafa's favor, is very biased because they have played so many more matches in clay rather than grass. Rafa leads there 13-2, Federer leads in grass 2-1, Rafa leads on hard courts 9-8. So, outside of their preferred surface, it is just 9-8 for Rafa.
Essentially, Rafa is a more dominant player in clay than Fed is on any surface, and there are so many more matches in clay than in grass, which makes the record so much in Rafa's favor.
But to be considered the greatest, you need to be great outside of your comfort zone too. And Fed is so much better than Rafa outside of their preferred surface.