I am not comparing across eras because that comparison eventually becomes futile. I compared the aggregate test wickets of Garner to Marshall and of Starc to Cummins. That is being relative to their own era. This is why Anderson (600 wickets) will always remain a superior bowler to say, Philander(200 odd wickets).
Garner was a support bowler who had the advantage of playing in a very strong team for most part of his career which is why he can't be rated as high as the Steyn's, Marshall's, Imran's and McGrath's. He was clearly not as good as them in test cricket which is visible by the fact that he had taken only 7 5-fers in 58 tests compared to almost twice the number of 5-fers taken by Starc in one test lesser and latter is still not over yet.
To me, Garner and Holding as a test bowler is at same level to another Windies legend, Andy Roberts and they are inferior to Marshall, Ambrose, McGrath, Steyn, Donald, WWI(for Pakistan), probably at same level as Shaun Pollock.
Garner though was a GOAT ODI bowler, much like Starc. Coming to that question, Cummins is superior to both of them and not just Starc but he will have to do it for 7-8 years more and end with 300+ test wickets. To become as good as McGrath or Steyn, he will have to take atleast 400 wickets and also other factors will come in because then comparison will be with absolute GOATs.
