seth I have to say that GD is a standard library since PHP 4.3. You have to actively compile PHP to not include it by using the —without-gd flag. All standard methods of installing PHP like yum on Linux or brew on macOS deliver the correct binaries including libs like GD unless you start tinkering with such flags. There are possibly systems where those flags to disable GD are predefined which I don’t know of.
While it can be challenging to set up Nginx servers, all required configuration to make rewriting work is included in the getting started guide — unless again, there are other issues with the server itself that are beyond the scope of a CMS documentation.
However — listing standard libraries as requirements would confuse many users. Since configuring a Nginx server on Linux obviously requires more knowledge than installing a classic LAMP stack on a Mac such as XAMPP, it seems to be quite obvious for me to install PHP including an image manipulation library when running a content management system.
