gmsh: 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/gmsh - from package: gmsh

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

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