Even if they end at 20 & 19, they are now in the same category to everyone. Some will consider Fed as GOAT and some Nadal as GOAT. You can see after the USO how the narrative has changed. Gone are the days where Federer was seen as "special", everyone now realizes that Nadal is up there (and he reached such numbers by playing less and despite more injury setbacks). Also long gone are the days when Nadal was being put down concerning "non-clay", he's solid everywhere and if anything Federer would also need another clay slam.
With a 1 slam difference it's like Court vs Serena vs Graf, each one could arguably have a claim (although for me Serena is the GOAT because Court's tally is inflated due to the weak AO and Graf's tally is inflated due to the Seles stabbing). Nadal has many more Masters 1000, the h2h, 2 slams on each surface, a god-like dominant surface, gold medal, etc. Federer has 1 extra slam, wtfs and for now weeks at no.1… but his low success in slams vs Nadal & Djokovic will always be a question mark.
I feel that if Djokovic ends at 16, even though he would always be part of the Big Three he will not have a claim as GOAT because a 3-4 slam difference is substantial. That being said, chances that they will end up 20-19-16 are low, I think Nadal has a few more great years in him and I can see him reaching 24 slams total and being the undisputed GOAT. Right now if you make a poll, most people would pick Nadal to end up with the slam record.