Over the past two months, new versions of this plugin have been released almost weekly. A quick search in Habr gave no mention of it, so I’ll 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 meta-plugin that is an implementation of the GREYC scripted image processing language.

Developers focus on the fact 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
• Sequences
• Spectral handling
• Various

Among these filters there are also Gimp's available in the standard package, but with advanced settings, which allows you to achieve better results.

For example, the familiar Unsharp Mask filter looks like this in G'MIC:



Well, since it's better to see it once than read it 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