So you manged to forget a grad filter and what you are shooting requires one. You could HDR the image, but yuck who wants to do that. Not me.
What you will need:
Anything that blocks light. Like your hand or a card.
Tripod (Don't even bother without one)
A cell phone with a stop watch will help.
Solution: You will want to do a long exposure in order to acheive this. Stop down to at least f16 or more. Your ideal exposure time should be about 2 seconds or more if you can do it.
First you want to meter for the sky. Say your lucky and it is one second (this won't happen). Next meter for the land, subject or what ever. Perfect, it meters for 2 seconds (what an ideal scenario).
Next you are going to block the area of the sky for a period of time while you take the exposure. Following our perfect scenario you will make a 2 second exposure in total. Setup with your clock in hand and place the card over sky portion with a slight over lap of the land. Cover the area for the first 1 second of the exposure. At that point move the card upwards in a sweeping motion, removing it form the image area (this will help create a more natural gradation).
Now this is not proper technique by any means. It will take a lot of tries before you are able to get acceptable results. But if you are stuck it can produce great results with a lot of patience, of course. Heck, pixels are free anyway.