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/bin/gc - from package: graphviz

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

On Debian possible default full paths are:

/usr/bin/gc - from package: graphviz

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

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