I believe the library you're using is here:
https://github.com/joomla/joomla-cms/blob/staging/libraries/cms/html/html.php#L567
So you can see in the parameter list that there is a param for $attribs
which is of the type array. The other thing is there is an additional $alt param you have to pass which might be your jtext just out of order. To pass that to the method you would do:
$html[] = JHtml::_('image', $grav_url, JText::_('PLG_CONTENT_AVATAR'), array('class'=>'img-circle'), true)
Or you could build the array outside the method as a variable:
$attribs = array();
$attribs['class'] = 'img-circle'; // I think this should work? haven't tested though.
$html[] = JHtml::_('image', $grav_url, JText::_('PLG_CONTENT_AVATAR'), $attribs, true)