You can modify the string in your language file. Remove the parts you don't want and add an extra placeholder for the new info. Modify one of the db queries to pull the sum(total_filesize), convert to GB, and add it into the strtr.
Code to change is in index.php
// Gather gallery statistics
if ($cat == 0) {
$result = cpg_db_query("SELECT count(*) FROM {$CONFIG['TABLE_ALBUMS']} as a WHERE 1" . $album_filter);
$nbEnr = mysql_fetch_array($result);
$album_count = $nbEnr[0];
mysql_free_result($result);
$sql = "SELECT count(*) FROM {$CONFIG['TABLE_PICTURES']} as p " . 'LEFT JOIN ' . $CONFIG['TABLE_ALBUMS'] . ' as a ' . 'ON a.aid=p.aid ' . 'WHERE 1' . $pic_filter . ' AND approved=\'YES\'';
$result = cpg_db_query($sql);
$nbEnr = mysql_fetch_array($result);
$picture_count = $nbEnr[0];
mysql_free_result($result);
$sql = "SELECT count(*) FROM {$CONFIG['TABLE_COMMENTS']} as c " . 'LEFT JOIN ' . $CONFIG['TABLE_PICTURES'] . ' as p ' . 'ON c.pid=p.pid ' . 'LEFT JOIN ' . $CONFIG['TABLE_ALBUMS'] . ' as a ' . 'ON a.aid=p.aid ' . 'WHERE 1' . $pic_filter;
$result = cpg_db_query($sql);
$nbEnr = mysql_fetch_array($result);
$comment_count = $nbEnr[0];
mysql_free_result($result);
$sql = "SELECT count(*) FROM {$CONFIG['TABLE_CATEGORIES']} WHERE 1";
$result = cpg_db_query($sql);
$nbEnr = mysql_fetch_array($result);
$cat_count = $nbEnr[0] - $HIDE_USER_CAT;
mysql_free_result($result);
$sql = "SELECT sum(hits) FROM {$CONFIG['TABLE_PICTURES']} as p " . 'LEFT JOIN ' . $CONFIG['TABLE_ALBUMS'] . ' as a ' . 'ON p.aid=a.aid ' . 'WHERE 1' . $pic_filter;
$result = cpg_db_query($sql);
$nbEnr = mysql_fetch_array($result);
$hit_count = (int)$nbEnr[0];
mysql_free_result($result);
if (count($cat_data)) {
$statistics = strtr($lang_list_categories['stat1'], array('[pictures]' => $picture_count,
'[albums]' => $album_count,
'[cat]' => $cat_count,
'[comments]' => $comment_count,
'[views]' => $hit_count));
} else {
$STATS_IN_ALB_LIST = true;
$statistics = strtr($lang_list_categories['stat3'], array('[pictures]' => $picture_count,
'[albums]' => $album_count,
'[comments]' => $comment_count,
'[views]' => $hit_count));
}
} elseif ($cat >= FIRST_USER_CAT && $ALBUM_SET) {
$result = cpg_db_query("SELECT count(*) FROM {$CONFIG['TABLE_ALBUMS']} WHERE 1 $current_album_set");
$nbEnr = mysql_fetch_array($result);
$album_count = $nbEnr[0];
mysql_free_result($result);
$result = cpg_db_query("SELECT count(*) FROM {$CONFIG['TABLE_PICTURES']} WHERE 1 $current_album_set AND approved='YES'");
$nbEnr = mysql_fetch_array($result);
$picture_count = $nbEnr[0];
mysql_free_result($result);
$result = cpg_db_query("SELECT sum(hits) FROM {$CONFIG['TABLE_PICTURES']} WHERE 1 $current_album_set");
$nbEnr = mysql_fetch_array($result);
$hit_count = (int)$nbEnr[0];
mysql_free_result($result);
$statistics = strtr($lang_list_categories['stat2'], array('[pictures]' => $picture_count,
'[albums]' => $album_count,
'[views]' => $hit_count));
} else {
$statistics = '';
}