gc.sh: 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/share/roundcubemail/bin/gc.sh - from package: roundcubemail

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

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