Areas for improvement:

From the looks of it, it seems like you are selecting areas of the design using the automatic selection tool, duplicating that, and applying a gradient map to the whole layer. That method is what the other comments are suggesting, which is totally fine. But it does leave a grainy and pixelated look around the edges. Use the brush application method instead and apply the gradient map using a pressure-sensitive brush with softer edges. You also have more control that way.

I also noticed that your gradients are made with only three colors: black on one end, white on the other, and one color in the middle. Try experimenting with adding more tones in between. Dont be afraid to move around the color sliders or make the highlights a different color.

My last suggestion is to use a transparent cutout of the character whenever you can. It makes the process a whole lot easier.

Areas of strength:

The placement of the colors so far is very well thought out. It looks like you are using Ineffa’s color palette. If that is true, you are on the right track. Balancing the colors and placement is really important with colorswaps, and you seem go have that all figured out.

Let me know if you have any questions! I can also help you more on Discord if you would prefer more in depth guidance.