Yes that's true, I'll amend my comment to say that there's nothing about HTML that prevents you from writing in this austere styling if that's all you want to do. Quickly scanning the gemtext intro, it looks like you have headings (three levels), links (but not inline), lists, blockquotes, and preformatted text. That's it. So with these tags you're there:

  <h1> <h2> <h3>
  <p>
  <ul> and <li>
  <a>
  <blockquote>
  <pre>

Granted gemtext markup is slightly more terse, but it would hardly matter in practice.