You'll need a program that makes animated gifs. There are expensive ones like Photoshop, or Flash.
Or there's free ones online in which you can convert it. You have to have many images with text. That's one way, that's the only way I know actually.
Example:
2 images, one with text, and one without. You use an online animation maker, decide the timing and the ending result is a flashing text animation.
There's ones you can just use to convert online. There are many on google (be careful which sites you click lol). I use Mcafee siteadvisor to avoid the dangerous sites. There's many marked "red" (aka dangerous).
Try this if you don't have Photoshop or other animation programs. It's safe from what I saw.
http://www.createagif.net/
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