koji-gc: 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/sbin/koji-gc - from package: koji-utils

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

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