It’s slightly complicated. The default editor in WordPress is what’s properly called the “block editor”, but while this editor was being developed it was known as the “Gutenberg” project. The development on Gutenberg was done in the form of a plugin, which people could install for the purposes of testing and contributing.
When WordPress 5.0 came around, all the functionality of Gutenberg was built directly into WordPress as the new default editor, called the “block editor”. So when you download and install WordPress, the built-in content editor is the block editor, and this is based on the original Gutenberg plugin, but it is not a plugin anymore.
However, there is still a Gutenberg plugin. This plugin is where development of future features of the editor is done, and can be installed separately as a way to preview, test and contribute to future features for the block editor.
If you want to go back to using the old editor, there isn’t a plugin that you can uninstall, but you can install the Classic Editor plugin, which will change the editor back to the default editor of WordPress prior to 5.0. Support for reverting back with this plugin is promised until 2022.