git - gitk: command not found

Question

Environment: Mac OSX 10.10. When I was trying to use gitk, it shows:

Coda-2:~ Coda$ gitk
-bash: gitk: command not found

I think the reason is the old version of git. Here is some information, and I have already typed brew update.

Coda-2:~ Coda$ git --version
git version 1.9.3 (Apple Git-50)
Coda-2:~ Coda$ brew install git 
Warning: git-2.4.0 already installed, it's just not linked

What is this message: it's just not linked?

Furthermore, I check the path of git:

Coda-2:~ Coda$ which git
/usr/bin/git

It should be in /usr/bin/local/git? How to fix it?

UPDATE

I figure it out, thank @TimCastelijns.

brew doctor said:

Warning: You have unlinked kegs in your Cellar
Leaving kegs unlinked can lead to build-trouble and cause brews that depend on
those kegs to fail to run properly once built. Run `brew link` on these:

    git

So, I typed:

Coda-2:~ Coda$ brew link git
Linking /usr/local/Cellar/git/2.4.0... 220 symlinks created

Then I typed gitk, and it worked and the path changed to /usr/local/bin/git. However, when I want to check the version of git. It still showed the old version, why?

Coda-2:~ Coda$ git --version
git version 1.9.3 (Apple Git-50)

Answer 1

I just installed the latest version of git via homebrew on macos.

11:28 $ git --version
git version 2.25.0

It doesn't seem to have gitk and git gui so I noticed the answer for linux and tried to install the same package git-gui via homebrew and it worked.

brew install git-gui

Found the bug here.

I'm running macos 10.14.5 Mojave.