Truncating article text
About the naming, and grouping your code. 

First improvement, change name of the variable from truncate to should_truncate, than you get

if should_truncate


instead of

if truncate


It is suggested to use methods to encapsulate code that is inside a block, for example code that is execute inside if statement, loops, or any other block of code. In your case

  def content(should_truncate)
    content = article.content
     if should_truncate
       content = truncate_article_content
     end
     content.gsub!(READ_MORE_REGEX, '')

     html = MarkdownService.instance.render content
     content_tag :div, html
  end

private

  def truncate_article_content
    index_at = article.content.index(READ_MORE_REGEX)
    if index_at
      truncate_length index_at
    else
      first_characters
    end
  end

  def truncate_length(length)
    url = _h.article_url(article)
    _h.truncate(article.content, length: length, omission: " [Show more](#{url})")
  end

  def first_characters(length= DEFAULT_LENGTH)
    _h.truncate(article.content, length: length)
  end