Create New Transparent Layer 200x200 (Or another size, depends what are you doing.)
Create Text layer. I need different gif for ever letter , but you can use one gif for whole text. Open gif you want to use as background for your Text:
Click Shift and Select First and Last frame in animation panel , so every frame will be selected.
Click little arrow in right top corner of animation panel and choose Copy Frame. Now we are back to our gif. Click that arrow again and choose Paste Frame.
Select Paste Over Selection and click OK.
Make Text layer as top layer in layer panel like in the picture.
This is how first frame looks. Text will be visible only for first frame. Choose Magic Wand Tool from Tools panel and click on the letter. (text layer must be active in layers panel). Click Select , then Inverse, so your gif must look like this:
Now choose First frame in animation panel and activate bottom layer and click Delete on your keyboard. Do same for every frame. And then simply delete Text layer. Click File -> Save For Web and Devices -> Save.
Thats our result: