I could solve this with a slight hack…

In Loadcomplete handler, we need to extract contents of the summary footer and add them to the group header row.

$(“#” + treeId + ” tr.jqfoot”).each(function () {

var summaryrow = $(this);

var level = summaryrow.attr('jqfootlevel');

var groupSelector = 'tr.jqgroup.' + treeId + 'ghead_' + level + ':first';

// get matching previous group row for level.

Searching a bit on the forum found /blog/?page_id=393/help/jqgrid-how-do-i-sum-the-values-in-columns-and-use-result/.