gvgen: 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/gvgen - from package: graphviz

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

On Debian possible default full paths are:

/usr/bin/gvgen - from package: graphviz

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

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