It has no inline formatting, only 3 levels of ATX headers (without trailing #s), one level of bullet points using only asterisk and not dash to delimit, does not merge touching non-whitespace lines (thus expecting one line per paragraph), and supports only triple-backtick fenced preformatted text areas that just flip on and off.
Maybe the biggest change is that links are necessarily listed on their own line, proceeded by a `=>` and optionally followed by alt-text.
My gemtext parser is maybe 70 lines and it is arguably 95% of what one needs from Markdown.