GFM is Github Flavored Markdown and is an extension of Markdown syntax. It should not be presumed to be supported except when writing markdown that will be displayed on Github. You have to ensure it is supported, otherwise. For example, if you are building a Markdown processing pipeline, you would need to incorporate it (like via the remark-gfm plugin).
A paragraph with a reference-style link.
A paragraph with [a reference-style link]. :
GFM supports footnotes via a reference-like syntax.
A[^1] sentence.[^2] [^1]: [^2]: which is also multi-line