gem64: 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/gem64 - from package: grass

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

Error in other languages:
gem64: bevel nie gevind nie
gem64: comandă negăsită
gem64: comando não encontrado
gem64: commande introuvable
gem64: command not found
gem64: käsku ei ole
gem64: không tìm thấy lệnh
gem64: komanda nerasta
gem64: Kommando nicht gefunden.
gem64: kommandot finns inte
gem64: komut yok
gem64: nie znaleziono polecenia
gem64: níor aimsíodh an t-ordú
gem64: no se encontró la orden
gem64: no s'ha trobat l'ordre
gem64: opdracht niet gevonden
gem64: parancs nem található
gem64: perintah tidak ditemukan
gem64: príkaz nenájdený
gem64: příkaz nenalezen
gem64: команда не найдена
gem64: командата не е открита
gem64: 命令找不到
gem64: コマンドが見つかりません