gdal-config: 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/gdal-config - from package: gdal-devel

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

On Debian possible default full paths are:

/usr/bin/gdal-config - from package: libgdal-dev

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

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