gnome-font-viewer: 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/gnome-font-viewer - from package: control-center

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

On Debian possible default full paths are:

/usr/bin/gnome-font-viewer - from package: gnome-font-viewer

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

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