Ubuntu 20.04 - gcc version lower than gcc-7

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:

1. 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

2. sudo apt update
3. 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.

The only solution that worked for me is:

Manual installing of .deb packages

(sad programmer noises)

1. Go to http://old-releases.ubuntu.com/ubuntu/pool/universe/g/
2. Download all .deb packages for gcc compiler version you want, f.e.:

gpc-2.1-3.4_3.4.6-6ubuntu5_amd64.deb
cpp-3.4_3.4.6-6ubuntu5_amd64.deb       lib32g2c0_3.4.6-6ubuntu5_amd64.deb
g++-3.4_3.4.6-6ubuntu5_amd64.deb       libg2c0_3.4.6-6ubuntu5_amd64.deb
g77-3.4_3.4.6-6ubuntu5_amd64.deb       libg2c0-dev_3.4.6-6ubuntu5_amd64.deb
gcc-3.4_3.4.6-6ubuntu5_amd64.deb       libstdc++6-dbg_3.4.6-6ubuntu5_amd64.deb
gcc-3.4-base_3.4.6-6ubuntu5_amd64.deb  libstdc++6-dev_3.4.6-6ubuntu5_amd64.deb

3. Manually install them by running command, f.e.:

sudo dpkg -i ./gcc-3.4-base_3.4.6-6ubuntu5_amd64.deb 
sudo dpkg -i ./cpp-3.4_3.4.6-6ubuntu5_amd64.deb
sudo dpkg -i ./gcc-3.4_3.4.6-6ubuntu5_amd64.deb

and so on...

Check the console output errors about package dependencies to figure out the package install order.

• If you encounter error (bug probably) about crossdependency of "g++..." package to "libstdc++..." package and v.v. then run install command to update libstdc++ package with exact version number, f.e.:

sudo apt-get install libstdc++6

4. Hooray! Use installed gcc (g++) version by, f.e.:

g++-3.4 -v

P.S.: of you're getting missing libs errors try

export LIBRARY_PATH=/usr/lib/x86_64-linux-gnu:$LIBRARY_PATH

before the build