It seems to me the code like following will solve your requirement:
jQuery("span.ui-icon-seek-first").removeClass("ui-icon").text("First");
jQuery("span.ui-icon-seek-prev").removeClass("ui-icon").text("Prev");
jQuery("span.ui-icon-seek-next").removeClass("ui-icon").text("Next");
jQuery("span.ui-icon-seek-end").removeClass("ui-icon").text("Last");
you can use jQuery("#gbox_" + id)
as a context of jQuery (second parameter of jQuery), where id is the id of the jqGrid (like "list") if you want modify paging buttons only for one selected jqGrid.