keep the grid view in a panel with scroll bars enabled, and did you try using ajaxtoolkit CollapsiblePanelExtender. It takes the id of the panel which we have to toggle,
<ajaxtoolkit:collapsiblepanelextender id="collapsible" targetcontrolid="IdPanelToCollapse" .....="" xmlns:ajaxtoolkit="#unknown"></ajaxtoolkit:collapsiblepanelextender>