compiling - Install gcc 5 on Ubuntu 18.04

I have gcc 7.3.0 on Ubuntu 18.04.
But I am working on a project where I need gcc less than version 6.
When I tried to install gcc 5.4 using sudo apt install gcc-5.4, I got this error:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package gcc-5.4
E: Couldn't find any package by glob 'gcc-5.4'
E: Couldn't find any package by regex 'gcc-5.4'  

What is the appropriate way to install gcc 5.4?

Thanks to @steeldriver, I could solve the problem as following:

Install gcc and g++

sudo apt install g++-5
sudo apt install gcc-5

Change the symlink to point to gcc 5 and g++ 5

sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-5 10
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-5 20
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-5 10
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-5 20
sudo update-alternatives --install /usr/bin/cc cc /usr/bin/gcc 30
sudo update-alternatives --set cc /usr/bin/gcc
sudo update-alternatives --install /usr/bin/c++ c++ /usr/bin/g++ 30
sudo update-alternatives --set c++ /usr/bin/g++

Credits: How to choose the default gcc and g++ version?