Giannis is clearly the better player, but I think the gap is mainly on offense, particularly Giannis' passing and better overall shot selection/getting to the line. I put them as roughly equal defenders given that Tatum is much more versatile and is a very high to elite level perimeter defender. Giannis simply can't guard on the perimeter for sustained periods, which is one of the reasons that we're a very difficult matchup for them and why even with a DPOY-level center Giannis is not able to bring that team higher than around 20th in defensive rating.
If Tatum's shot selection was better and he was more aggressive taking it to the rim instead of relying so much on the jumper, he could really challenge for the best player in the league status right up there with Giannis. He just hurts himself so much by constantly not taking smart shots and falling back on difficult, iso jumpshots.