gpsbabel: 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/gpsbabel - from package: gpsbabel

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

On Debian possible default full paths are:

/usr/bin/gpsbabel - from package: gpsbabel

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

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