gitk: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/gitk - from package: gitk

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/gitk - from package: gitk

You may install the required package with command apt-get install {package}

Error in other languages:
gitk: bevel nie gevind nie
gitk: comandă negăsită
gitk: comando não encontrado
gitk: commande introuvable
gitk: command not found
gitk: käsku ei ole
gitk: không tìm thấy lệnh
gitk: komanda nerasta
gitk: Kommando nicht gefunden.
gitk: kommandot finns inte
gitk: komut yok
gitk: nie znaleziono polecenia
gitk: níor aimsíodh an t-ordú
gitk: no se encontró la orden
gitk: no s'ha trobat l'ordre
gitk: opdracht niet gevonden
gitk: parancs nem található
gitk: perintah tidak ditemukan
gitk: príkaz nenájdený
gitk: příkaz nenalezen
gitk: команда не найдена
gitk: командата не е открита
gitk: 命令找不到
gitk: コマンドが見つかりません