We overload the meaning of "GFM" to mean "GitLab Flavored Markdown", which is a superset of GitHub's version. However it can cause confusion as they are not the same thing.

Standard Markdown Extended Markdown in GitLab

If you’re working in spreadsheet software (for example, Microsoft Excel, Google Sheets, or Apple Numbers), you can copy from a spreadsheet, and GitLab will paste it as a Markdown table. For example, suppose you have the following spreadsheet:

See the documentation for HTML::Pipeline’s SanitizationFilter class for the list of allowed HTML tags and attributes. In addition to the default SanitizationFilter allowlist, GitLab allows span, abbr, details and summary elements.

You can add a table of contents to a Markdown file, wiki page, or issue/merge request description, by adding the tag [[_TOC_]] on its own line. It will appear as an unordered list that links to the various headers.