The best alternative to GNOME is KDE Plasma, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to GNOME and five of them are so hopefully you can find a suitable replacement. Other interesting alternatives to GNOME are Xfce, Cinnamon, Lomiri and Bluetile.
Plasma is a cross-device work environment by the KDE Community where trust is put on the user's capacity to best define their own workflow and preferences.
Xfce (pronounced as four individual letters) is a free software desktop environment for Unix and other Unix-like platforms, such as Linux, Solaris and BSD. It aims to be fast and lightweight, while still being visually appealing and easy to use.
Lomiri is the open source mobile and desktop environment that adapts to you computing needs. Supported on all kinds devices.
Bluetile is a tiling window manager for Linux, designed to integrate with the GNOME desktop environment. It provides both a traditional, stacking layout mode as well as tiling layouts where windows are arranged to use the entire screen without overlapping.
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KDE make two desktops that are GNOME alternatives: KDE 4 and its successor Plasma 5. Both are popular, extensible and compatible with Qt much more than with GTK. KDE is known for having a huge, huge amount of options built right in (rather than accessed via extensions, like in GNOME). It's also a more "classic" desktop experience - people coming from Windows will find it more familiar than they will find GNOME.