First, never touch a gradient again when you're doing pixel art.
Alright, now, one thing you're missing is contrast. What shade of background are drawing on? If it's relatively dark, you probably won't see how light your dark values are.
Now here we go-

1. This is the original. You're using lots of colors. 10 if I didn't miss any. As you can see-
2. -6 of those 10 aren't necessary. This edit is the exact same as the original but with the colors merged together, still using only colors that were already there. Use as few colors as you can, it makes it look cleaner and keeps coherence.
3. Your shading is flat. In this case, a pumpkin is basically a sphere made of sphere-like shapes. So shade round things just like you would shade spheres, and definitely don't keep your dark tones for outlines.
4. Make it satisfying just to look at. You have 10 colors, and they're all so close together that it makes it look even flatter. Have fun with your palette. Don't be afraid to hue shift farther than you think you should, and try to keep your darkest value in the lower quarter of shades.
Those are a small fraction of a fraction of the basics, and it'll help, but I'd suggest you go find some tutorials for beginners. There's one that I've been looking for, it explains basically all the basics like antialiasing lines, simple shading, banding and all that. Maybe someone has the link?