gfsplot: 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/gfsplot - from package: gerris-mpi
/usr/bin/gfsplot - from package: gerris

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

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