Problems getting a working installation of gsl 2.6

You can build GSL 2.6 on 18.04 LTS from sources by using commands below:

sudo apt-get update
sudo apt-get install build-essential dpkg-dev debhelper

cd ~/Downloads
wget -c http://archive.ubuntu.com/ubuntu/pool/universe/g/gsl/gsl_2.6+dfsg.orig.tar.gz
tar -xf gsl_2.6+dfsg.orig.tar.gz
cd gsl-2.6+dfsg/
wget http://archive.ubuntu.com/ubuntu/pool/universe/g/gsl/gsl_2.6+dfsg-2.debian.tar.xz
tar -xf gsl_2.6+dfsg-2.debian.tar.xz
rm gsl_2.6+dfsg-2.debian.tar.xz

dpkg-buildpackage -uc -us
sudo apt-get install ../*gsl*.deb

to get

$ gsl-config --version
2.6

and then try to compile your application.