On using Gradient map, you need to first understand how Gradient map works on both GIMP and PS. What "Gradient map" does is that the program will detect the Base image's colour composition and apply the Gradient map from lighter parts to deeper colour parts respectively.
So take lovekaurt's WW gradient map for example, the gradient map is made to apply a light tone skin(WW) to a deep colour tone skin character (Vixen). What you want to do, is:
-Finding out on the BASE skin, what is the threshold for program decide to progress into the deeper colour parts. And match the Colour Stops (black arrows) to that threshold accordingly.
See the following two Gradient map for example, (1) is for character specifically with Vixen's deep colour composition, while the (2) one is for character with light colour tone.
View attachment 3215158
*you still need the black colours for Vixen's, but it's an old set of images I had in an old conversation, lots of correction and things I'd learnt since then. You ABSOLUTELY dont need that many Colour stops in a well tested Gradient Map, it's just GIMP's testing process makes it hard to find where exact is the threshold. My PS Gradients looks like this
View attachment 3215215
Result:
View attachment 3215160
The process is a lot of trial and error even on PS, as Lovekaurt mentioned. PS's gradient is more easy to understand and real time applies can save yourself from your own temper a LOT.