I've been using this technique for awhile now and thought some others might like to give it a try
it's 5 steps to turn this:

into this
and for reference this is an example of a real graduated filter-
THIS ONE IS NOT MY PICTURE
Step 1:
open up the desired image in photoshop and create a new blank layer
Step 2:
Pick the two colors you want for the filter effect. I chose a blue for the sky and a goldish orange color for the sand.
Step 3:
drag the gradient tool across the blank layer untill it it matches the general area you want the colors to be in.
Step 4:
Now change the layer style from "normal" to "overlay"
Step 5:
POOF! it's pretty much the same result as a graduated filter. From this point you can adjust the gradient layers saturation to adjust the intensity of the effect or erase parts of the gradient layer to remove the effect or lessen it in other parts of the image
Well there yah go, there might be better ways to do this but this is the method i worked out and it seems to work out pretty well for me. Hope it helps