When you try to save a GIF with multiple layers, you should get the option: "Save as Animation." When you select that, you will get some options. You want to select "Replace Layer." It should be in the bottom right of the menu that pops up.
There should also be an option to set the time between frames. If you already have a time specified on a layer, it will not override it. (If you have a time by a layer, it will show on the layers menu on the main screen. You can delete the time the same way you would change the layer name.)
Its been a while since I played with GIMP though. I don't have it on my computer right now, so I can't double check the proccess exactly.
Hope that helps.
Though, I wouldn't recommend an animation that fast. It would get annoying pretty quick. However, if you save it and open it in GIMP all you have to do is edit the time between frames. You can edit the text layer anyway you like without it messing.