You need to have the pictures you want animated (say, a picture with one black word and the same picture but with the word colored white, so that it'll flash back and forth between colors) and put them in one new image, on different layers. Then open up the Layers dialogue, and rename the layers according to the length of time you want each frame to stay up. For example: Layer 1 (100 ms)
Now that layer, when saved as a GIF, will stay visible for that length of time. (And I want to say that those are milliseconds, but don't quote me on that.) You can make each layer the same length or have certain frames stay up longer, shorter, whatever you want. The bigger the number, the longer it'll stay visible in your animation.
This might take some experimenting, probably some tinkering with the timing, so to check how your animation looks, go to Filters > Animation > Playback and the animated image will pop up in a little box that shows what the finished product will look like. (But it doesn't automatically reflect changes made to the layers, so if you check your play-back and then need to make changes, you'll have to close and then open it back up.)
Any layers that don't have the time set will not show up until they do, and when you save, you'll be given the option of putting one set time (that you pick) on all the time-less layers.
Actually, If you just want a quick, dirty animation, I guess you can just skip the time-setting part, and jump to saving your multi-layered image as a GIF, then when it prompts to choose between flattening and animating, pick animation and choose one time to be applied to all of the layers. That could work alright. Personally I prefer to do it manually, since you can try different effects with the timing, do more complex animations and stuff, but yeah, it's more time-consuming than just a simple all-(100 ms) animation. So mess with a little, and whatever you think will work best for you, I'd stick with that.
