What is the intention that you have and the underlying problem that you would like to solve?

To implement the minimap as can be seen in sublime text http://www.sublimetext.com/

the problem is a lack of a mimimap feature? As I said this is an enhancement not something that's broken

You should probably check out https://github.com/jonocodes/GeditTextmap, I'm not sure it's being maintained.

The needinfo here is why this feature is useful? Apart from copying Sublime text.

You can already add marks to a document, with the Bookmarks plugin, to easily navigate through the document. You have line numbers in the margin, and with the scrollbar you know where you are in the document.

There can also be more specific features to easily navigate through a document, like a list of functions for a programming language, or the document structure (list of chapters etc) for LaTeX, etc.

I understand that the minimap can be more visual. For small documents it is not a big problem to navigate through the document. It is more a problem for big files, and the visualization for big files is not really useful since the minimap is quite small.

I set NEEDINFO in comment 1 because requesting a feature without any explanation of intention is not a convincing reason to implement it. Removing NEEDINFO as an answer was provided afterwards.

There is a minimap plugin at

https://github.com/ddev/textmap

(I have not tested it)

Done:
https://blogs.gnome.org/nacho/2015/05/10/minimap-for-gedit/