If it's saved as a gif, there will not be any layers there. Only a psd file has layers as already mentioned. If there is text in the image and you want to alter it, you might be able to use the rubberstamp tool to copy portions of the image to fill in the background where the text is. That way your starting with a fresh surface. Then select the type tool, font, color, etc and type in what you want to say.
Be sure to before starting altering the image, change modes to rgb so you can keep your layers intact. That way you can save the file first as a psd and if you don't like your changes when you flatten the image and save as gif, you can just open the psd file..
Hope that helps
Slaughter22