gfan_mixedvolume: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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/gfan_mixedvolume - from package: gfan

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