gmtswitch: 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 CentOS possible default full paths are:

/usr/bin/gmtswitch - from package: GMT

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

On Debian possible default full paths are:

/usr/lib/gmt/bin/gmtswitch - from package: GMT

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