gd extension php ubuntu

will usually find the correct package and version for your install setup and just install everything

sudo apt-get update

sudo apt-get install php-gd
sudo service apache2 restart

For 5.6 PHP

sudo apt-get install php5.6-gd

For 7.0 PHP

sudo apt-get install php7.0-gd

For 8.0 PHP

sudo apt-get install php8.0-gd