These are the options exposed in cmark-gfm.h:

Include a data-sourcepos attribute on all block elements.
OPT_SOURCEPOS

Render softbreak elements as hard line breaks.
OPT_HARDBREAKS

Render raw HTML and unsafe links (javascript:, vbscript:, file:, and data:, except for image/png, image/gif, image/jpeg, or image/webp mime types). By default, raw HTML is replaced by a placeholder HTML comment. Unsafe links are replaced by empty strings.
OPT_UNSAFE

Render softbreak elements as spaces.
OPT_NOBREAKS

Validate UTF-8 in the input before parsing, replacing illegal sequences with the replacement character U+FFFD.
OPT_VALIDATE_UTF8

Convert straight quotes to curly, --- to em dashes, -- to en dashes.
OPT_SMART

Use GitHub-style <pre lang="x"> tags for code blocks instead of <pre><code class="language-x">.
OPT_GITHUB_PRE_LANG

Be liberal in interpreting inline HTML tags.
OPT_LIBERAL_HTML_TAG

Parse footnotes.
OPT_FOOTNOTES

Only parse strikethroughs if surrounded by exactly 2 tildes. Gives some compatibility with redcarpet.
OPT_STRIKETHROUGH_DOUBLE_TILDE

Use style attributes to align table cells instead of align attributes.
OPT_TABLE_PREFER_STYLE_ATTRIBUTES

Include the remainder of the info string in code blocks in a separate attribute.
OPT_FULL_INFO_STRING