You need to update your system:

sudo apt update
sudo apt upgrade
sudo apt install gcc

I'm still on Ubuntu 22.04 LTS but needed g++14. The sudo apt-get gcc-14 did not work for me, as it installed clang++14 for some reason (perhaps a misconfiguration on my part). What did work for me was following the instructions I found at https://www.dedicatedcore.com/blog/install-gcc-compiler-ubuntu/

The steps I took:

sudo apt install build-essential
sudo apt install libmpfr-dev libgmp3-dev libmpc-dev -y
wget https://ftp.gnu.org/gnu/gcc/gcc-14.1.0/gcc-14.1.0.tar.gz
tar -xf gcc-14.1.0.tar.gz
cd gcc-14.1.0
./configure -v --build=$(uname -m)-linux-gnu --host=$(uname -m)-linux-gnu --target=$(uname -m)-linux-gnu --prefix=/usr/local/gcc-14.1.0 --enable-checking=release --enable-languages=c,c++ --disable-multilib --program-suffix=-14.1.0
make
sudo make install

And if you would like to make it the default:

sudo update-alternatives --install /usr/bin/g++ g++ /usr/local/gcc-14.1.0/bin/g++-14.1.0 14
sudo update-alternatives --install /usr/bin/gcc gcc /usr/local/gcc-14.1.0/bin/gcc-14.1.0 14

After that, g++ showed I was running version 14.1.0. I was then able to compile my project that included some c++20/23 features that were not in the previous versions of g++ (chrono/format).

Tested on Ubuntu 20.04:

sudo add-apt-repository ppa:ubuntu-toolchain-r/ppa
sudo apt update
sudo apt install g++-10

I wanted to install gcc-6 alongside my existing installation of gcc-9 and this is how I did it. First off, sudo apt install gcc-6 didn't work because the package wasn't found so I had to add a new repository that contained gcc-6. To do this, I first found a repository that contains gcc-6 from Google and ended up at: https://packages.ubuntu.com/bionic/gcc-6

From there, I chose an architecture (amd64) which took me to a page with all the mirrors. I added the first mirror (mirrors.kernel.org/ubuntu) to /etc/apt/sources.list and did sudo apt update and then installed gcc-6 with sudo apt install gcc-6.

To switch between gcc versions, I used the following:

sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-6 6
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-9 9

sudo update-alternatives --config g++

gcc-12 is not available in ubuntu 20.04, so we need to compile it from source code, here are the steps which I borrowed from this video:

• Step 1: clone gcc source code and checkout gcc-12 branch

$ git clone https://gcc.gnu.org/git/gcc.git gcc-source
$ cd gcc-source/
$ git branch -a
$ git checkout remotes/origin/releases/gcc-12

• Step 2: make another build dir

Note this is important as running ./configure from within the source directory is not supported as documented here.

$ mkdir ../gcc-12-build
$ cd ../gcc-12-build/
$ ./../gcc-source/configure --prefix=$HOME/install/gcc-12 --enable-languages=c,c++

• Step 3: installing GCC prequisites and run configure again

The missing libraries will be shown in above ./confgiure output, search and install them one by one.

$ apt-cache search MPFR
$ sudo apt-get install libmpfrc++-dev
$ apt-cache search MPC | grep dev
$ sudo apt-get install libmpc-dev
$ apt-cache search GMP | grep dev
$ sudo apt-get install libgmp-dev
$ sudo apt-get install gcc-multilib
$ ./../gcc-source/configure --prefix=$HOME/install/gcc-12 --enable-languages=c,c++

An alternative is to run the download_prerequisites script.

 cd gcc-source/
$ ./contrib/download_prerequisites
$ ./../gcc-source/configure --prefix=$HOME/install/gcc-12 --enable-languages=c,c++

• Step 4: compile gcc-12

$ make -j16

Still flex is missing:

$ sudo apt-get install flex
$ ./../gcc-source/configure --prefix=$HOME/install/gcc-12 --enable-languages=c,c++
$ make -j16
$ make install

Another way is to use Ubuntu 22.04 where gcc-12 is available. In Ubuntu 22.04, gcc-12 can be installed with apt:

$ sudo apt install gcc-12

To install gcc-7 on ubuntu you should use this ppa:

sudo add-apt-repository ppa:jonathonf/gcc
sudo apt-get update

Then run: apt-cache search gcc-7

sample output :

gcc-7-base - GCC, the GNU Compiler Collection (base package)
gcc-7 - GNU C compiler
gcc-7-multilib - GNU C compiler (multilib support)
gcc-7-plugin-dev - Files for GNU GCC plugin development.
gcc-7-test-results - Test results for the GCC test suite
lib32gcc-7-dev - GCC support library (32 bit development files)
libgcc-7-dev - GCC support library (development files)
gcc-7-doc - Documentation for the GNU compilers (gcc, gobjc, g++)
gcc-7-hppa64-linux-gnu - GNU C compiler (cross compiler for hppa64)
gcc-7-locales - GCC, the GNU compiler collection (native language support files)
gcc-7-source - Source of the GNU Compiler Collection
libx32gcc-7-dev - GCC support library (x32 development files)
gcc-7-offload-nvptx - GCC offloading compiler to NVPTX
lib64gcc-7-dev - GCC support library (64bit development files)

Install gcc-7:

sudo apt install gcc-7