gems-server: 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
/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/gems-server - from package: gems

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