First of all you'll need to modify the actual qa-db-selects.php file to change the qa_db_posts_basic_selectspec(...) function - putting your new version in qa-theme.php won't work because this isn't a function in the theme class.
Second if you want it before the 'what' span, you're best off doing this in two stages. First, pass the email through to the theme by adding this line somewhere towards the end of qa_post_html_fields(...), again by modifying the original qa-app-format.php:
Then, do something like this in your qa-theme.php file:
class qa_html_theme extends qa_html_theme_base
function post_meta($post, $class, $prefix=null)
$prefix="YOUR GRAVATAR HTML WHICH USES $post['email']".$prefix;
qa_html_theme_base::post_meta($post, $class, $prefix);
This is basically prepending the HTML to show a Gravatar to $prefix (if the email is not empty) then calling through to the standard post_meta(...) theme function, which outputs the $prefix before the rest of the meta info.