Working with all kinds of designers and illustrators, we sometimes get assets that aren't ideal for importing into After Effects. The client can get pretty mad when the look of the video doesn't match the look of the design boards. Gradient Maps are the common culprit. Here's how to convert a Gradient Map to LUT
After Effects Tips: Gradient Layer to LUT
First off, I don't mean to shame gradient maps, I use them all the time. Gradient maps are an awesome way to unify a design and explore stylistic looks in Photoshop.
Here's an example. I wanted to make this background texture red, but with some dark crimson values, a hotter red midpoint, and a lower white point.
So I added a gradient map.
The problem with gradient maps is that they don't import to After Effects. It just comes in as some blank weird layer. So you have to convert it a LUT (short for Lookup Table), first. To do this the right way you have to create a new video layout file (same specs as what you are working on). This is important because it needs a white BG layer for it to export properly. I haven't gotten it to work right when you right-click on the gradient map and duplicate it into a new file (because there is no BG layer, and adding one didn't work correctly). So after you open the new file, copy or duplicate your gradient map into it. Then got to File>Export>Color Lookup Table. Leave everything at default and name your LUT and save it to someplace in your project folder where it makes sense (since After Effects needs to reference it).
After that hop on over to After Effects and import, your .psd file. You'll see your Gradient Map come in blank, just like this example.
Then you need to add an adjustment layer over the layers you want it to affect. Add the Effect Apply Color Lut to the adjustment layer. Then choose the file where you saved your LUT. It doesn't matter much in this instance which of the file formats you choose from your exported LUT. I used .3DL in this example. After that, you'll see your Gradient Map applied perfectly as a LUT over the assets/footage that you wanted treated.
Hope that helps. For more vital tips check out this one on layer masks: http://attackmotiondesign.com/after-effects-tips-how-do-i-apply-an-effect-only-to-a-masked-area-in-adobe-after-effects