As mentioned in the comment section, GCC versions lower than 7 are still available in xenial repository. You can follow the following steps to install gcc-5:
Add xenial to /etc/apt/sources.list
Open sources.list with sudo
sudo vim /etc/apt/sources.list
Add the following lines in the sources.list file
deb http://dk.archive.ubuntu.com/ubuntu/ xenial main
deb http://dk.archive.ubuntu.com/ubuntu/ xenial universe
sudo apt update
sudo apt install g++-5 gcc-5
Reference answer: Install gcc 4.9 at ubuntu 18.04
To change default gcc version to gcc 5 you can follow this link. Below I am adding the steps for completeness.
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-5 5
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-5 5
And then select the correct version manually as below:
sudo update-alternatives --config gcc
sudo update-alternatives --config g++
If you find this answer helpful, please also consider upvoting the reference answer from which most of my answer is borrowed.
https://askubuntu.com/questions/1235819/ubuntu-20-04-gcc-version-lower-than-gcc-7