WARNING: This tutorial is my longest so far, and it may be confusing, but any GIMP user should be able to follow it, if you can't PM me or post.
This one seems like an important tutorial to do, because as you can see in many signatures, text outline is commonly used. There are so many ways to do this, but here is the way I do it.
1: Type the text..
Type the text, and after you have typed it, look on the text toolbar, and click "Path from Text", if you don't have that option, click on the text again.
2: Select the text..
Now go to the text image, and click (Select --> From Path).
3: Grow the selection..
Now we must grow the selection, which now is of the text, so we can add our outline. You must go to (Select --> Grow). From there, you can set how many pixels you want your outline to be, I suggest 3-5.
4: Create a new layer..
Go to (Layer --> New Layer) then click ok.
(in this picture, is doesn't show that I have grown the selection, but yours will)
5: Fill with a color..
Go to (Edit --> Fill with FG Color), make sure your Foreground color, is what you want your outline to be, if it isn't change it before you do this.
6: Repeat of 2..
Now go to the text image, and click (Select --> From Path).
(I took this image from the second step, don't pay attention to the text behind it)
7: Feather..
Go to (Select --> Feather), and feather it by pixels, I am using 50, you may want to go lower. NOTE: After doing this, your selection may disappear, this is normal if using a large number, don't worry about it!
8: Cut..
Go to (Edit --> Cut) or CTRL-X.
9: Remove background..
If you don't want a white background, go to (Dialogs --> Layers) or CTRL-L. Click on the background, and change the opacity to 0.
(This was taken from another guide, the layer names will be different, the background will remain)
If you did the last step, when saving you must add a ".png" to your name, this way it will save it as a PNG, you can use ".gif" instead of PNG, because it does allow transparency, but I don't recommend it.