gsmsendsms: 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/gsmsendsms - from package: gsm-utils

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

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