aco to gif conversion is not so much as a file conversion, but more of applying color swatches / palettes in Adobe Photoshop to opened or new pictures. Once this done, you can save the pictures to one of the available formats, including GIF.
You can find dozens of articles about working with color swatches in Photoshop, including Adobe help files. YouTube is also a great source of guides and tutorials in case you struggle with .aco files.