Gravity: 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/Gravity - from package: GeographicLib

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

On Debian possible default full paths are:

/usr/bin/Gravity - from package: geographiclib-tools

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

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