What is the battery type number?

The model number for the battery is the: CR2450, 3V.

On the picture shown, even the gums changed color. How can that be ...? Teeth got whiter and gums now look pinker. Does it lighten gums too?

If you are referring to the before and after pic, it's hard to see gums from both arches in each picture. The gel is not supposed to bleach your gums, but if you use too much gel and over load the trays, it can cause a temporary dehydration to occur, and make them look lighter. It's only temporary, but can happen do to user error. It can also occur if you leave the gel on for too long, as it will mix with saliva, causing it to run off onto the gums. The teeth whitening gel should only whiten your teeth when used correctly.

How often can you use it? Daily?

We recommend whitening your teeth once daily for up to a week, but it does not have to be a set routine. It may take less than a week, so just save your teeth whitening gel, and do maintenance treatments on an as-needed basis.

Does this really even work?

As long as you follow the directions, it really does work. I've used the whitening strips before, and I think I like this product better as I did not have to worry about the tray moving around, like the strips. And actually I saw results faster with this than with the strips.