The Gradient Map interface looks difficult to use, but with a few pointers, you’ll find it surprisingly easy to use. While you can apply a Gradient Map directly to a layer (Images > Adjustments > Gradient Map), I recommend you apply Gradient Maps as adjustment layers (Layer > New Adjustment Layer > Gradient Map) to take advantage of both the greater flexibility and control you’ll gain over the final effect.
Once activated, there are a number of default presets you can experiment with, but it’s most likely that you’ll want to create your own. Simply click on an existing gradient in the Properties panel to activate the Gradient Editor. Click New. Click at the bottom of the gradient to add new colors. A pointer will appear; double-click it or the Color box to choose a color. You can move the pointer to direct the color into different tonal values. (Move left to target darker values and right to target lighter values. Alternately, enter a new number in the Location field.) The diamonds left and right of it will control how each color fades into surrounding colors. You can add dozens of different pointers/colors, but for most applications, I recommend you restrain yourself to as few as possible. You can delete a pointer/color by clicking on it and clicking Delete or by pressing the Delete key. When you create an effect you’d like to use more than once, type a Name and click Save; you can easily store, retrieve and share these GRD files.
The color effects you can generate with the Gradient Map are so powerful and so varied, you simply must spend a little time experimenting with it to truly understand both how far you can go and how subtle you can get. Consider this kind of visual research time well spent.
After you’re done experimenting, then it’s time to deliver. Working with the Gradient Map often takes a little finessing. You’re likely to be a little disappointed if you try and get the perfect colors with the Gradient Map alone. You can spend a great deal of time picking and re-picking colors until you get it just right. Instead, try working more broadly, getting close to a desired effect and then fine-tuning the results.