Problem

Currently the code block macro does not support gherkin language.
 

Suggested Solution

Add support for it.
 

Why This Is Important

Gherkin language is used by a number of customers, it would make the user experience better if gherkin was added a native option to the code block macro.
 

Workaround

The below workaround only works for Confluence older than v9, since custom language addition has been removed in Confluence 9. refer to Improved Code Block Macro Security

At the moment it is possible to add additional languages manually

The Code Block macro uses syntaxhighlighter brushes in the background, in order to add a new language you just need to find a suitable syntaxhighlighter.js file.

A search for a suitable gherkin file produced this however please be aware that this is from a third party site and has not been verified by Atlassian and you may want to check if there are other more suitable options.

I installed it locally and tested with some sample code, this is how it was presented when using the code block macro:

Alternatively a custom file for gherkin could be written using the following documentation:
https://github.com/syntaxhighlighter/syntaxhighlighter/wiki