How can I create a table of content in GitLab wiki?

So this exists! I finally found a Merge Request in in the GitLab Community Edition: Replace Gollum [[_TOC_]] tag with result of TableOfContentsFilter

As its name describes, to have a table of contents you need to write the following:

[[_TOC_]]

All together, you can write something like:

This is a page for my project

[[_TOC_]]

## Credits

bla bla

## License

bla bla

and will show like this:

This is available from the GitLab 8.6 release as described in its milestone.

Should be something like this

 ##### Table of Contents  
[1.-First Topic](#first-topic)
[2.-Second Topic](#second-topic) 
[3.-Third Topic](#third_topic)

And then you will reference each on the same page by using

 <a name="first-topic"/>
 <a name="second-topic"/>
 <a name="third-topic"/>

This should be easier with GitLab 15.3 (August 2022):

Visualize table of contents in the WYSIWYG wiki editor

When you’re editing a wiki page in the rich text WYSIWYG editor, you expect to see an accurate representation of the content as it will appear after it’s published.
Some elements are too complex for plain text formatting, and require short codes or macros that generate content when the page is built.

A wiki’s table of contents is one example: it generates a list from your page’s subheadings, simplifying navigation of very long pages.
However, WYSIWYG page editing displayed a table of contents as a static placeholder block.

GitLab 15.3 shows a visual representation of the Table of Contents in the WYSIWYG wiki editor.

To add a table of contents while editing a page, select the plus (+) icon in the toolbar, then select Table of Contents.
The table of contents updates in real time as you create and edit subheadings on the page, helping you monitor the outline of your longer wiki pages.

See Documentation and Issue.

Try using {{< toc >}} on top of the document. It should automatically generate the TOC for you.

You can use either [TOC] or aforementioned [[_TOC_]] as described on the link.