"If GML was an infant, SGML is the bright youngster who far
   exceeds expectations and made its parents too proud, but
   XML is the drug-addicted gang member who had committed his
   first murder before he had sex, which was rape." -- Erik Naggum

GML is from the late 1960s and is, arguably, easier to read. SGML from the 1980s expanded upon this. XML is essentially just an offshoot of SGML and much less readable than SGML is. For example, here is a valid SGML document:

  <anthology>
    <poem>
      <title>The SICK ROSE
      <stanza>
        <line>O Rose thou art sick.
        <line>The invisible worm, 
        <line>That flies in the night
        <line>In the howling storm:
      <stanza>
        <line>Has found out thy bed
        <line>Of crimson joy:
        <line>And his dark secret love 
        <line>Does thy life destroy.
    <poem> 
      <!-- more poems go here    -->
  </anthology>

Aside from XML's lineage, there is no shortage of markup languages available with varying levels of readability.