gfal-sum: 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 CentOS possible default full paths are:

/usr/bin/gfal-sum - from package: gfal2-util

You may install the required package with command yum install {package}

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