tg: 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/tg - from package: topgit

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

On Debian possible default full paths are:

/usr/bin/tg - from package: topgit

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

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