gtcli: 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 Debian possible default full paths are:

/usr/bin/gtcli - from package: gtg

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

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