Mays should be on the list. But the answer is Babe Ruth (who, BTW, spent the first 4 years of his career being, principally, a very good pitcher).
Lots of guys deserve to be on the list ahead of Gwynn. Schmidt? Aaron? Mantle? Joe Morgan?
A better question is who was the 2nd best hitter of all times? In my book that is Williams. Mays and Hornsby and Musial, right behind.
Williams gave up his '43-'45 seasons when he enlisted. If you give him the average of '42 and '46 (quite similar seasons) over those three years, he would have another 540 hits, 111 homers and 450 walks. During the years from '41-'49 he was averaging getting on base nearly 340 times a year. HIs OBP was over .500 during that run. Arguably, his 4-yr run from '41-'47 was as good as any 4-yr run that Ruth had (impacted by Ruths injury year on '22). From '41-'47, Williams' SLUMP year was '48, when he went .369-.497-.615. Some slump!
Hornsby had a 5-yr run where his lowest batting average was .384, btw. We forget about him.
BTW, some consideraton should be given to Josh Gibson and other Negro League stars, like Oscar Charleston. Finding complete records for those players is impossible, of course. But an extensive search of newspapers for Box Scores shows that over 1570 PA (career compilation) Gibson hit .359-.413-.646. that compares to Musial's .331-.417-.559. Records, alas, are amazingly incomplete. In essence, we shall never know exactly how good a Gibson or Charleston really was. Let's just say, REALLY good.