Code
<? $set_table="0"; ?>
<table cellpadding="5" border="0">
{exp:gallery:categories gallery="{gallery_name}"}
<?
$set_table = $set_table +1;
if ($set_table == "1") {echo"<tr>";} ?>
Insert other tags here.
<? if ($set_table == "2") {echo"</tr>"; $set_table="0";} ?>
{/exp:gallery:categories}
</table>