transmission-gtk: 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/transmission-gtk - from package: transmission-gtk

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

On Debian possible default full paths are:

/usr/bin/transmission-gtk - from package: transmission-gtk

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

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