gnibbles: 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/gnibbles - from package: gnome-games-extra

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

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