If an attempt is made to save an RGB image as a filetype that does not support more than 256 colors, this dialog window will be presented to the user. Each filetype will have a slightly different dialog layout, but the dialog clearly illustrates the action the user must take to rectify the situation. The export options for GIF export are shown below.
Exporting a file will not modify the original image in any way.
GIF can only handle layers as animation frames
When exporting to GIF, any layers contained in the image may be used as frames in an animation. If this is not the desired result, this option should be enabled.
If the image is being saved as an animation, this option should be chosen.
GIF can only handle grayscale or indexed images
This option should be used when saving the image as a color GIF. Manual conversion can be achieved by using the Convert to Indexed feature.
If this option is chosen, the image will be converted to a grayscale indexed image before saving.