The term GOAT is often misused. It stands for Greatest Of All Time, which intends to express ALL TIME literally, not only "of one specific era". In tennis, however, we do use this expression almost like a pet thing. My favorite, is ma'goat... And even when someone tries to apply it more technically, they often forget all what happened before the Open Era (1968) or even Rankings Era (1973).
But do they do it? Easy answer: to crown their favourite tennis player as the GOAT of the sport (# magoat). There is no other reasonable explanation for it, seriously. The biggest achievements in tennis have varied throughout the time, so must the criteria used to determine it. For example, if Grand Slams are one of the best measures to determine "GOATness" than it must also include Pro-Slams and Championships, since these 2 competitions were the ones featuring the best athletes in the world.
There is, however, some experts who herald the passage of time and dressing fashion style of the game. Whoever defends that "times were different", thus less important, keep in mind that your own option gets old year after year, and unless you want to become utterly irrelevant in the future, you better change your mind. For those saying that "tennis with pants" are not a real sport, I usually give the advice of pursuing a career in haute-couture (in which Milan hosts a Grand Fashion Slam event). Little they know that in 20-30 years time, people will laugh at the Wimbledon's oppressive white dressing code, concluding as irrelevant all these majors won by Sampras and Federer simply because they dressed in white...
We are not Kronos, the titan-god of time. Time does not freezes during our existence and erases the past before us. The 1960s were the new tennis in regard of Pre-War Era. The 1930s were the new avant-garde version of tennis in respect of the Championships Era of before 1925. Past greats were greats in the time they lived in. Otherwise we may presume Jesse Owens and Mark Spitz were slow and incompetent athletes. Worse yet, we may consider the Roman Empire or the Samurai warriors as lame and weak, since they could not even defeat the least modern army/soldier in nowadays world. It's wrong to judge the past based on present standards, and it will still be wrong in the future.
Still, if we want to erase it so badly, and say that titles do not really matter, because the way certain players play nowadays is endlessly superior to past ones; then we may simply forget about titles rankings and rivalries and focus only on exhibitions matches. No one does to tennis what Kyrgios does, best of all times unquestionably.
In regard of the greatest players, if we really want to consider Grand Slams instead if majors (incl. Pro-Slams and Championships) and weeks as #1 instead of Year-Ends (since we do not have a proper weekly rankings prior to the 80s) then we CANNOT call them GOAT, but at most the greatest of modern times (GOMT) or Open Era (GTOP - Greatest Tennist of Open Era).
In the actual GOAT raking we may be surprised by the Top20 in history:
Laver
Djokovic
Tilden
Federer
Rosewall
Nadal
Sampras
Pancho
Lendl
L. Doherty
Connors
Wilding
Budge
Borg
Larned
McEnroe
E. Vines
Kramer
Lacoste
Cochet
This Top20 GOAT Rankings does acomplish an excellent work in really digging deep to the ATGs of the sport. Let's not forget that rivalries once accounted as much as Grand Slams for the sport, since the recognition of PR media and prize money resided more on "Tours" than in the old out of fashion tradiiotnal Slams tournaments. Davis Cup direct H2H confronts and Championships encounters (played in 3 surfaces rather than the only 2 of the old Salms) were the gauges of greatness during much of the history of the sport.
GOAT acronym is reserved not only to the greatest players of all times, but also for those who deeply know the sport itself and respect the history of tennis. Next week, we will not be born again...