If you're playing in the Open Era - when particularly from mid-'80s on there are no professional or political reasons for missing slams - and other Open Era greats have won 10+ slams, and slams are by far the most important measure of greatness in the sport, then it is a prima facie conclusion that you do indeed need at least 10 slams to be in the GOAT discussion. This is effectively a fact, & it is a philosophical mistake to imagine that a fact must be proved beyond any shadow of doubt. Applying that criterion, there would be no facts (except perhaps in logic & mathematics), so whether a claim was a fact would be irrelevant.
If you are disputing the natural prima facie conclusion that as an Open-Era player you need 10+ slams to be in the GOAT discussion, it is your job to state what other achievements would make up for this relative deficiency.
Obviously, you do not
know the difference between fact and opinion. A fact is a statement that can be proven true or false. An opinion is an expression of a person’s feelings that can not
be proven. The GOAT discussion is matter of opinion because, for example, it includes debate about level of competition which can not
be proven true or false.
Do you have any proof that he lost the argument?
When debating any issue, there is an implicit burden of proof on the person making an allegation of fact. Andy1402 made an allegation of fact but failed to provide proof.