First, add the ubuntu-toolchain-r/test PPA to your system with:
sudo apt install software-properties-common
sudo add-apt-repository ppa:ubuntu-toolchain-r/test
Install the desired GCC and G++ versions by typing:
sudo apt install gcc-7 g++-7 gcc-8 g++-8 gcc-9 g++-9
The commands below will configure alternative for each version and associate a priority with it. The default version is the one with the highest priority, in our case that is gcc-9.
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-9 90 --slave /usr/bin/g++ g++ /usr/bin/g++-9 --slave /usr/bin/gcov gcov /usr/bin/gcov-9
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 80 --slave /usr/bin/g++ g++ /usr/bin/g++-8 --slave /usr/bin/gcov gcov /usr/bin/gcov-8
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-7 70 --slave /usr/bin/g++ g++ /usr/bin/g++-7 --slave /usr/bin/gcov gcov /usr/bin/gcov-7
Later if you want to change the default version use the update-alternatives command:
sudo update-alternatives --config gcc
There are 3 choices for the alternative gcc (providing /usr/bin/gcc).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/bin/gcc-9 90 auto mode
1 /usr/bin/gcc-7 70 manual mode
2 /usr/bin/gcc-8 80 manual mode
3 /usr/bin/gcc-9 90 manual mode
Press to keep the current choice[*], or type selection number: You will be presented with a list of all installed GCC versions on your Ubuntu system. Enter the number of the version you want to be used as a default and press Enter.
The command will create symbolic links to the specific versions of GCC and G++.
What if I want install gcc-9.5.0? This install gcc-9.3 by default.