Php7.3-gd is outdated and probably removed from default linux repositories. You should install the version 7.4. Try this command:
sudo apt update && sudo apt install php7.4-gd
Can't install php7.3-gd on ubuntu 20.04
Php7.3-gd is outdated and probably removed from default linux repositories. You should install the version 7.4. Try this command:
sudo apt update && sudo apt install php7.4-gd