gerris2D: 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/gerris2D - from package: gerris-mpi
/usr/bin/gerris2D - from package: gerris

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

Error in other languages:
gerris2D: bevel nie gevind nie
gerris2D: comandă negăsită
gerris2D: comando não encontrado
gerris2D: commande introuvable
gerris2D: command not found
gerris2D: käsku ei ole
gerris2D: không tìm thấy lệnh
gerris2D: komanda nerasta
gerris2D: Kommando nicht gefunden.
gerris2D: kommandot finns inte
gerris2D: komut yok
gerris2D: nie znaleziono polecenia
gerris2D: níor aimsíodh an t-ordú
gerris2D: no se encontró la orden
gerris2D: no s'ha trobat l'ordre
gerris2D: opdracht niet gevonden
gerris2D: parancs nem található
gerris2D: perintah tidak ditemukan
gerris2D: príkaz nenájdený
gerris2D: příkaz nenalezen
gerris2D: команда не найдена
gerris2D: командата не е открита
gerris2D: 命令找不到
gerris2D: コマンドが見つかりません