GOATS do not trail their biggest rival 2-5 in slam finals (2-6 if you add a semifinal).
That is a lop-sided tally though having to play the best clay courter of all time on clay for 3 of those 5.
The best player of all time (whoever that is) on either gender would beat their biggest rival on every surface. Graf beats Seles and Navratilova on every surface in the slams and elsewhere. Navratilova destroyed Chris Evert, the greatest female clay courter ever in the 84 French Open final. Evert has beaten Navratilova at every slam, including her beloved Wimbledon (ok semis not final but still).
Laver has bested Rosweall and Gonzales on every slam and on every surface, including a straight sets win in the French Open final over Rosewall. Sampras wasnt owned by any opponent, despite his failure to win the French. He has even beaten most of his eras greatest clay courters at the French head to head though- Muster, Courier, Bruguera. Borg beat McEnroe and Connors on every surface, and beat both at the U.S Open, just never got the 7th win to win the title. Budge beat Perry and Riggs on every surface and won the Calender Slam.
Federer has never beaten Nadal at the French. Nadal has beaten Federer everywhere. In the last year Federer at only 26/27 has lost 3 straight slam finals to his biggest rival on 3 different surfaces, and the one on Nadal's surface was trounced. Sorry that isnt GOAT material.
In the open era, who would you put ahead of Federer? Federer has been the best on everything but clay -> where he has been second best for 4-5 years, second best only to the arguably best ever clay courter. He's 5-4 against Nadal at non-clay and yes he's something like 2-9 against Nadal on clay and yes the past few slam final losses to Nadal have not been that good, that said, except for clay, he's pretty darn even to Nadal and you can't say Nadal is better than Federer on hards or grass (yet).
Ok and my GOAT is very much limited to the open era only, even more specifically post 70s, this is my personal opinion/ choice because for me that's the only realistic comparison time period. Clearly this is not the case with a lot of people - most people in fact, since many including yourself consistently include guys like Laver, Rosewall, Emerson etc etc. I am not discrediting them by any means but I'm putting pre-70s in a whole different category.
So essentially we are looking at Federer who in the last 40 years is only the second man to win all four majors (Agassi) and one of the few who has won slams on all surfaces (Agassi, Nadal come to mind, anyone else?). 14 majors.. and 5 major finals: 6 at W, 4 at F, 4 at AO, 5 at USO. Add to that he's been arguably the best for 5 years on grass, hards, indoors, you name it and second best on clay. Agreed he might not be your ideal goat but would you put anyone above him? I can't put either Sampras or Borg above that, perhaps Borg comes close if he could have even won USO once... but that's about it.