gtbl: 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/gtbl - from package: groff

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

On Debian possible default full paths are:

/usr/bin/gtbl - from package: groff-base

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

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