How can I unlock an Index layer in Photoshop?

I scanned a piece of handwriting as a PNG document. I then opened it in Photoshop CS4 and, under Save for Web and Devices, optimized the image (which included removing some ugly red lines that were on the original paper). I then saved the document as a gif.

WhenI opened the new gif file, however, I discovered that the image is now on a locked layer, named "index." How can I unlock this layer -- or can I?

As I mentioned above, I removed some reds in the original image when I optimized it. I'd like to create a new background layer in the gif that will fill the transparent lines that were created when I removed the colors. If i can't unlock the new gif's "index" layer, can anyone advise me on how to create a new background layer/color?

Thank you.