McGriff's most comparable player is Rafael Palmeiro. Before being busted for use of performance-enhancing drugs, Palmeiro was considered a shoo-in Hall-of-Famer. His career batting numbers are very similar to McGriff's:
McGriff:
284/377/509, 134 OPS+
Palmeiro:
282/371/515, 132 OPS+
(OPS+ is a measure of how good a player’s on-base+slugging percentage is relative to the rest of the league, when adjusted for difficulty of the player's home ballpark.)
These numbers are basically identical. Over his career, Palmeiro averaged 33 HR and 105 RBI for every 162 games played; McGriff averaged 32 HR and 102 RBI. The main difference between the two is that McGriff peaked early, during a pitcher-friendly era, while Palmeiro peaked later, during a hitter-friendly era.
There are some other differences, of course. Palmeiro was superior on defense, McGriff was superior in postseason. McGriff was equally good on the road and at home, while Palmeiro hit better at home than on the road. Mostly, though, the difference is in counting stats - Palmeiro reached two "magic numbers" - 3020 hits and 569 home runs. This is mostly a function of playing time - Palmeiro made 20% more plate appearances in his career, about half of them as a DH. This is not trivial, and it does give Palmeiro a clear edge. However, is it enough of an edge that he was seen as a cinch Hall of Famer while McGriff is viewed as an also-ran? I don't think so.
And that was before Palmeiro was outed as a user of PEDs, something of which McGriff has never vbeen accused. So, how do we consider Fred in light of the PED issue?