As in Linux and most Unix(like) operating systems, the graphical interface is not the default way to interact with the operating system, contrary to what happens in Windows and OS X, there are many “desktop environments”. Desktop environments are a cohesive collection of graphical applications to interact with the operating system and GNOME is one of them, alongside KDE and XFCE, which are the most famous ones (but there are a lot more).

This is GNOME:

This is KDE:

This is XFCE:

Gnome is one of the most popular desktop environments on Linux, it can be said to look most similar to MacOS

Is a free and open-source desktop environment for GNU/Linux operating systems. https://wikiless.org/wiki/GNOME?lang=en

To add on to the other replies, GNOME is the simple and polished desktop experience for linux. KDE aims for features and maximum customizability with a modern desktop paradigm, XFCE aims for lightweight and classic desktop paradigms, and GNOME aims for a simple and fresh take on desktop paradigms with a focus on creating sensible defaults in place of customization.

GNOME is generally slow to adopt new features compared to KDE but when they do it’s more thought out and integrated, if that makes sense. It also has a more decisive path forward, it’s development team has a vision and actively works toward that vision despite what others might want which might be a good thing or a bad thing depending on what you expect out of your desktop experience.

It’s design is a remnant of early 2010s desktop design where touchscreens and 2in1s were the future and everyone was racing to redesign their desktop to work with both but at this point it’s the only one that succeeded and got close to that end goal

It’s a desktop environment for free/libre operating systems.