I am using latest version of jqGrid(4.8.2), and there are some strange:
There is no file: grid.custom.js, in folder(or in github)
In wiki(http://www.trirand.com/jqgridwiki/doku.php?id=wiki:methods) that method must be in file grid.custom.js
Finally, in source i can find method: gridUnload : function ( jqGridId ), and it's look similar(but you must call this method like this:
$.jgrid.gridUnload('jqGridKoeff');
But this situation is really strange. Be careful!