Saving Images as GIFs in GIMP

Files you work on in GIMP are saved in XCF, GIMP's native file format that allows you to build up images with multiple layers. But you may want to save your image in a different format when you've finished working on it. For example, a GIF file might be appropriate if you’re using a simple graphic in a web page. GIMP can be used to produce GIF files with these easy steps.

You can use either Save as and Save a copy from the File menu to save a file as a GIF. They do basically the same thing, but using Save a copy will save a whole new file while keeping the XCF file open in GIMP. Save as will automatically switch to the new GIF file.
Click on Select File Type in the dialog box just above the Help button. Select GIF image from the list of file types.

The Export File dialog will open if you're saving a file with features that aren't supported by GIF, such as layers. Unless you've specifically set up your file to be an animation, you should select Flatten Image.
GIF files use an indexed color system with a maximum limit of 256 colors. If your original XCF image contains more than 256 colors, you’ll be offered two options. You can Convert to indexed using default settings, or you can Convert to grayscale. In most cases, you'll want to select Convert to indexed. You can click the ​Export button when you've made the necessary selections.

This next step is very simple as long as you're not saving an animation. Select Interlace. This will produce a GIF that loads progressively, but it's unnecessary in most cases. The other option is to add a GIF comment to the file, which could be your name or information about the image that you might need in the future. Click the Save button when you’re happy.

You can now use the GIF version of your image on a web page. If you want to make any changes, you can return to the XCF version, make your amends, and resave it as a GIF file.
If your GIF results in a poor quality image with lots of spots and obvious areas of different colors, you might be better off saving your image as a JPEG or PNG file. GIFs aren’t suited for photo-type images because they're limited to supporting only 256 individual colors.