Install gcc 6 on Ubuntu

ubuntu-install-gcc-6

sudo apt update && \
sudo apt install build-essential software-properties-common -y && \
sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y && \
sudo apt update && \
sudo apt install gcc-6 g++-6 -y && \
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-6 60 --slave /usr/bin/g++ g++ /usr/bin/g++-6 && \
gcc -v

I'm on 20.04 and get
E: Package 'gcc-6' has no installation candidate
E: Package 'g++-6' has no installation candidate

1. sudo vim /etc/apt/sources.list
2. add deb http://dk.archive.ubuntu.com/ubuntu/ bionic main universe to file
3. sudo apt update
   4.sudo apt-get install gcc-6 g++-6 -y

for those who encounter the error:

E: The repository 'http://dk.archive.ubuntu.com/ubuntu bionic InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.

consider adding [trusted=yes]:

deb [trusted=yes] http://dk.archive.ubuntu.com/ubuntu/ bionic main universe

After that, I can install them successfully~

I could'nt add the key for some reason but adding bionic as a trusted source as you suggested worked. thanks!