I talk about gradient maps so much that I probably should elaborate a little on their function and my use. Besides, I've got a little time on my hands at 30,000 feet and hours to spare.
In Photoshop you can find gradient maps via Layer>New Adjustment Layer>Gradient Maps. I like making these adjustments as new adjustment layers because they don't affect your base layer (the one with the image) but instead adds the adjustment as an effect. This should appease those that stress about losing "information" contained within the original image.
Anyway, I don't have any "textbook" information about gradient maps. All I can tell you is how I've seen it used and how I use it. I have never done any research about it online because it comes pretty easy at the expense of several hours of trial and error.
I think my first contact with gradient maps resulted from wanting to create a B&W image with more "punch" (contrast). Basically after converting the image to B&W via the Black and White layer adjustment (see my previous post), I'd add the B&W gradient map to really push the contrast. If you haven't done this you should try it. It really adds significant contrast to the image and might even have to be dialed down. The good thing is that you can adjust the "Smoothness" to control the effect. I rarely touch the smoothness these days and instead opt to control the amount of B&W gradient map via opacity. This however assumes that you've done the B&W conversion otherwise adjusting opacity might bring back some color to the image which you may or may not want.
Lately I haven't done any 100% B&W images. Most of them are desaturated via a B&W conversion and/or a B&W gradient map. My brother says all pictures look better a little desaturated and that it's a trick. I guess I'd have to agree. It's really a one-size-fits-all. I think it's harder to pull off a vibrant and colorful image than it is to make it desaturated. I bring this up because I'm saying that (these days) I don't use the B&W gradient map so much for the B&W elements but for the luminosity elements that the B&W gradient map delivers. So what I do is apply the B&W gradient map and then instead of leaving it on normal blending, I use "luminosity" blending. To my knowledge (I could be wrong) the luminosity blending provides the well... (light) "levels" contrast of the B&W elements and applies that to the image while filtering out the monochromatic element of the B&W gradient map. Essentially you get all the contrast of a B&W gradient map with all of the original colors.
I rarely leave any adjustment layer at 100% opacity. I am compelled to adjust things so I feel in control. Besides, whenever I leave anything at 100% I get the subconscious feeling that I've "maxed out" and what if I needed more? Could I get more? Maybe I haven't gone far enough? So I'd rather have gone too far and then dial back with opacity.
So that about sums up my use of the B&W gradient map, but what about all these other colorful presets that we have in Photoshop? Well, I say go ahead and play with them. To me the presets are great and I've used quite a few of them myself. Each one adds a different feel and effect. If you want to see some of them in action check out my most recent posts particularly those of Ashley. Sometimes I can't seem to get the right "feel" and I'll experiment with different colors of my own but typically the presets provide great benchmarks.
Along with the color gradient maps, I like to experiment with the blending modes. Especially with the color gradient maps, I feel like the right blending mode makes a big difference. Are you going for subtle? Or over the top? Do you want the added contrast or just the colors? I've used the following blend modes (off the top of my head): soft light, overlay, color, multiply. I'm sure there are others but those are just the ones that I remember that I typically try when applying a gradient map.
To bring this post full circle, I started using gradient maps when I discovered significant highlight and shadow coloring in pictures of those that I admire. Sometimes it's particularly obvious that there's a green or purple tint to the entire image. Sometimes it's a two-tone highlight and shadow tint (incidentally that Lightroom performs very well!). And sometimes it's a rainbow of colors like the Yellow, Violet, Orange, Blue which I might have used before, I can't remember. Not to sound like a broken record but each map has its own feel and depending on the colors of the original image you're working with will create a different effect... so there's no right answer and you'll just have to experiment to get the effect you want. I've seen gradient maps applied to lots of wedding photography to give it that "old vintage" look and sometimes I feel like it's overplayed. For me, it's a new foray into creating an effect/feel so I'll suffer through the cliche-ness for now.
The last thing I'll add about gradient maps is that it can sometimes subtract contrast to your image. Sometimes it makes it hazy and if you're trying to create a crisp and vibrant image, this might not be the way to go. That's when you can consider different opacities and blends. Hell I think I've even used a gradient map that wasn't a solid "gradient type" but instead I tried using noise (one of Ashley's pictures).
Happy experimenting :) If you have questions, I'll be happy to answer.