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

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

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