gsl-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/gsl-config - from package: gsl-devel

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

On Debian possible default full paths are:

/usr/bin/gsl-config - from package: libgsl0-dev

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

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