add an event handler for button click in a gridview

I have many ideas, but the solution depends on whether or not you want to do a postback with this. If you don't want to do a postback, I'd use jQuery to manage this in the click event handler. There are many examples showing how to do this, but here's a sample of the type of syntax you'd be looking at:

$('#myId').click(function() {
  $('#myOtherButton').attr('disabled','enabled');
});

[EDIT]
The OP wanted further information. In case you don't know, jquery is a javascript framework that makes developing client side functionality an absolute breeze. You need to download the jquery library from JQuery[^] and add it into your web page as an include. Then, you'd hook up to the javascript I showed above, replacing #myId and #myOtherButton with the relevant ids inside a script that is fired when your page loads. The click event is then hooked up to the button.