gem2.1: 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/gem2.1 - from package: ruby2.1

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

Error in other languages:
gem2.1: bevel nie gevind nie
gem2.1: comandă negăsită
gem2.1: comando não encontrado
gem2.1: commande introuvable
gem2.1: command not found
gem2.1: käsku ei ole
gem2.1: không tìm thấy lệnh
gem2.1: komanda nerasta
gem2.1: Kommando nicht gefunden.
gem2.1: kommandot finns inte
gem2.1: komut yok
gem2.1: nie znaleziono polecenia
gem2.1: níor aimsíodh an t-ordú
gem2.1: no se encontró la orden
gem2.1: no s'ha trobat l'ordre
gem2.1: opdracht niet gevonden
gem2.1: parancs nem található
gem2.1: perintah tidak ditemukan
gem2.1: príkaz nenájdený
gem2.1: příkaz nenalezen
gem2.1: команда не найдена
gem2.1: командата не е открита
gem2.1: 命令找不到
gem2.1: コマンドが見つかりません