The hardest part is probably generating the gradient image in the first place, since I don't think either MSPaint or Paint 3D can do it easily. Luckily, there's a free program called "Paint.net" ( https://www.getpaint.net/ ) that has a nice interface and comes with a gradient tool. Download, install, and run it, and press Ctrl-N (or File->New) to create a new image at 1920x32 px (or whatever your monitor's width is). Then use the Colors panel to select a primary and secondary color for your gradient. Then use the Gradient tool to drag a gradient from left to right over the blank canvas:
Save that image, then load up Curtains and create a new style (or copy an existing one). Then under the Taskbar section, select Horizontal Taskbar, pick your image, and Apply Changes. You now have a gradient taskbar! If they are light colors, you'll probably also want to change the default taskbar mode to Light under the Colors tab so that Windows knows to use a dark color for the taskbar text.