To perform a click, cast the GeckoNode type to GeckoHtmlElement then you can call the click method.
GeckoHtmlElement element = (GeckoHtmlElement)gWB.Document.GetSingleElement(x.ToString());
if (element != null)
{
element.Click();
}
Setting the value can be different depending on the type of the element, but you can try and use attributes such as NodeValue, TextContent, and InnerHtml.