In the past two months, new versions of this plugin are released almost weekly. A quick search on Habra did not give any mention of it, so I will try to fill this gap.
Project Page - gmic.sourceforge.net
Binary versions of the plugin for Linux, MacOS, Windows are available for download.
G'MIC is a metaplagin, which is the implementation of the GREYC scripting image processing language.

Developers emphasize that everyone can write their own filters using this framework. But inside the plugin there is also an impressive gallery of ready-made filters, more than enough for everyday work with images. The current version 1.3.9 includes about 180 filters grouped into the following categories:

• Arrays & Frames
• Artistic
• Colors
• Contours
• Deformations
• Degradations
• Enhancement
• Layers
• Lights & Shadows
• Patterns
• Presets
• Rendering
• Sequence
• Spectral handling
• Various

Among these filters are found and available in the standard Gimp'a, but with advanced settings that allows you to achieve better results.

For example, the usual Unsharp Mask filter in G'MIC looks like this:
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Well, since it is better to see once than read ten times - this is the result of some other filters.

Original

Artistic - Cartoon

Patterns - Paper

Arrays & Frames - Pattern Frame

Sequences - Polaroid

Lights & Shadows - Relief Light

Presets - Thin brush smoothing