There is no gitk in git 2.26.0 which is installed in Mac OS X 10.15.4
I have to move git back to 2.23 with steps below
brew unlink git
cd homebrew-core
cd Formula
git checkout d04b60b58 git.rb
brew install git.rb
git --version
gitk
High resolution support.
$ brew cask install retinizer
open /System/Library/Frameworks/Tk.framework/Versions/Current/Resources/
Drag Wish and drop into Retinizer
