Lol Djokovic Federer Nadal have all compared the Olympics to a slam. However none consider the WTF anywhere near a slam. I’ll take their word over yours thank you very much.
Alright, let's do this.
Consider the players who have won Olympic Gold: Murray x2, Nadal, Massu, Kafelnikov, Agassi, Rosset, Edberg, Mecir.
That's eight players, and only three 6+ Slam winners. In other words, Nadal is only one of three "great" players to win Olympic Gold.
Now consider eight players who haven't (only players who were elite level in 1988 or later, when Olympics tennis re-started): Federer, Djokovic, Sampras, Becker, Wilander, Lendl, McEnroe, Courier.
Hmm...which is a better group of players?
The Olympics happens every four years. Because of that, not all great players win it. Now is it a nice and impressive accomplishment? Absolutely. But again, it isn't a factor in GOATdom, except as part of the big title total.
Now consider this list of 6+ Slam winners who haven't won an ATP Tour Finals: Nadal, Wilander.
And the list of 6+ Slam winners who have won it: Connors, Borg, Lendl, McEnroe, Becker, Edberg, Sampras, Agassi, Federer, Djokovic.
In other words, of the 12 players who have won 6+ Slams in the Open Era, what we could call "all-time greats," Nadal is one of only two who haven't won the Tour Finals, and the only one of five 10+ Slam winners. All of the other GOAT candidates (10+ Slam winners)--Borg, Sampras, Federer, Djokovic--have all won it at least TWICE.
So yeah, I'd say the lack of a World Tour Finals is a much bigger hole in a GOAT resume.