But GB can clearly take on many high tiers at once.
If you're fighting 6 high tiers at once at an advantage you're not slightly stronger than them, you're at least a tier higher.
Yes but I did not say he was slightly higher.
As an example, Law or Zoro will be able to on 7 scabbards at once yet they aren't top tier even though they are a tier above high tier.
In my view, top tiers are able to give each other at least very high diff...hence why the Yonkou refrained from fighting each other. Also why Akainu took 10 days to deal with Aokiji. Also how WB lost half a face to Akainu.
However, looking at Greenbull's portrayal this chapter with his fear of Shanks, do you think he will be able to push Akainu to extreme diff like Aokiji/ fight WB? I don't think so