gfscombine2D: 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/gfscombine2D - from package: gerris-mpi
/usr/bin/gfscombine2D - from package: gerris

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

Error in other languages:
gfscombine2D: bevel nie gevind nie
gfscombine2D: comandă negăsită
gfscombine2D: comando não encontrado
gfscombine2D: commande introuvable
gfscombine2D: command not found
gfscombine2D: käsku ei ole
gfscombine2D: không tìm thấy lệnh
gfscombine2D: komanda nerasta
gfscombine2D: Kommando nicht gefunden.
gfscombine2D: kommandot finns inte
gfscombine2D: komut yok
gfscombine2D: nie znaleziono polecenia
gfscombine2D: níor aimsíodh an t-ordú
gfscombine2D: no se encontró la orden
gfscombine2D: no s'ha trobat l'ordre
gfscombine2D: opdracht niet gevonden
gfscombine2D: parancs nem található
gfscombine2D: perintah tidak ditemukan
gfscombine2D: príkaz nenájdený
gfscombine2D: příkaz nenalezen
gfscombine2D: команда не найдена
gfscombine2D: командата не е открита
gfscombine2D: 命令找不到
gfscombine2D: コマンドが見つかりません