gfscompare2D: 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/gfscompare2D - from package: gerris-mpi
/usr/bin/gfscompare2D - from package: gerris

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

Error in other languages:
gfscompare2D: bevel nie gevind nie
gfscompare2D: comandă negăsită
gfscompare2D: comando não encontrado
gfscompare2D: commande introuvable
gfscompare2D: command not found
gfscompare2D: käsku ei ole
gfscompare2D: không tìm thấy lệnh
gfscompare2D: komanda nerasta
gfscompare2D: Kommando nicht gefunden.
gfscompare2D: kommandot finns inte
gfscompare2D: komut yok
gfscompare2D: nie znaleziono polecenia
gfscompare2D: níor aimsíodh an t-ordú
gfscompare2D: no se encontró la orden
gfscompare2D: no s'ha trobat l'ordre
gfscompare2D: opdracht niet gevonden
gfscompare2D: parancs nem található
gfscompare2D: perintah tidak ditemukan
gfscompare2D: príkaz nenájdený
gfscompare2D: příkaz nenalezen
gfscompare2D: команда не найдена
gfscompare2D: командата не е открита
gfscompare2D: 命令找不到
gfscompare2D: コマンドが見つかりません