gthumb: 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/gthumb - from package: gthumb

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

On Debian possible default full paths are:

/usr/bin/gthumb - from package: gthumb

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

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