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GIMP allows you to put a white outline around text.
1) right click the text layer
2) select "alpha to selection"
3) go to the "select" tab (top left)
4) click "grow"
5) select how many pixels you want to grow it by (this is the width of the resulting outline around your text)
6) make a new layer
8) use the fill tool in that layer on the "grown" selection you just made.
9) make sure that the text layer is above the "white outline" layer.
...
10) profit!
GIMP is a free tool to use, but a hard tool to master.
You can do most (like 95%) of what photoshop does but it's either slower or less intuitive.
(But I use it anyway)