Both .aco and .grd files come for Photoshop and are used to alter images, either with color swatches or gradients, but it is not possible to convert these files between each other, simply because they contain different kind of settings. At best, you can use both to alter same image.
You can find more about color palettes, swatches and gradients in various Photoshop guides and tutorials, some are even on Youtube.
Updated: October 5, 2023