gfscompare3D: 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/gfscompare3D - from package: gerris-mpi
/usr/bin/gfscompare3D - from package: gerris

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

Error in other languages:
gfscompare3D: bevel nie gevind nie
gfscompare3D: comandă negăsită
gfscompare3D: comando não encontrado
gfscompare3D: commande introuvable
gfscompare3D: command not found
gfscompare3D: käsku ei ole
gfscompare3D: không tìm thấy lệnh
gfscompare3D: komanda nerasta
gfscompare3D: Kommando nicht gefunden.
gfscompare3D: kommandot finns inte
gfscompare3D: komut yok
gfscompare3D: nie znaleziono polecenia
gfscompare3D: níor aimsíodh an t-ordú
gfscompare3D: no se encontró la orden
gfscompare3D: no s'ha trobat l'ordre
gfscompare3D: opdracht niet gevonden
gfscompare3D: parancs nem található
gfscompare3D: perintah tidak ditemukan
gfscompare3D: príkaz nenájdený
gfscompare3D: příkaz nenalezen
gfscompare3D: команда не найдена
gfscompare3D: командата не е открита
gfscompare3D: 命令找不到
gfscompare3D: コマンドが見つかりません