Extra Thoughts

 If there are missing areas of a photo you may beable to get them back. Sometimes easily, sometimes it may take a bit of ingenuity on your part as well.
I find usually a missing eye can be selected & copied from the other opposite eye. It needs to be done as a layer though, as it will need to be flipped. Remember faces are not symetrical. You can get away with it more on other parts of the body as long as you have people standing/sitting straight on.
Buttons, medals etc can be done with the clone brush if there are some already in the photo.
Sometimes one side of a body, usually only a small area (maybe a neck, collar and arm including part of the chest can be selected then copied as a new layer, flipped and put in the place where the other side should be. You can also do this with legs. Before merging down erase any extra away of the new layer as possible (If your new layer is selected only it will be affected).
Where appropriate I have even used photos from the net to fill in missing parts and also the blur tool and smudge. 
If you intend to use thes photos online only use creative commons or public domain photos though.
Whenever making a selection to copy an area, always feather it at least by 3 pixels (more depending on the size) to give it a smooth merge.  You still may need to use the heal tool to soften the joins.

Pop

 There are a few ways to pop photos. You will find the best one for you. Try the Exposure  slider under colour. Also the shadow & highlights filter. In GMIC there is a special filter to pop shadows, under lights and shadows. Contrast as said will sharpen your photo, as well as making it darker & brighter it also makes the colours more enhanced. I rarely use the saturation filter since I found this out.

Here is a tutorial to make shadows pop

This beginner’s tutorial describes how to make shadows pop in GIMP without that overly cheesy “HDR” look.

1. Open a photo in GIMP. In this example I’m using a plain ol’ JPEG.

2. Duplicate the layer and desaturate it.

3. Invert it.

4. Set blending mode to “Overlay”, strength 50.

5. Duplicate the base layer again, move it to the top and set blending mode to “Lighten only”.
   A side-effect of this lame tone mapping technique is that high radius halos mighty appear, and the sky could be darkened in a cheesy way. You would more easily notice halos in what should be an even blue sky than in grass or rocks. As we only want to make the shadows pop and we don’t want a fake “HDR look” cheesy sky, we fix it in this step. This layer’s job is to undo the effects of this whole technique in the light areas.

6. Go back to the black-and-white layer and blur it. How much you blur it depends on how large your image is and on taste, that’s why I left this step for last. Try 30 and go from there.
   Tutorial by Morgan Hardwood

Denoise.

 This should be your last thing to do to a photo. But you may find things to do after your finished.

There is a denoise in gimp under enhance but many more in GMIC under repair.

Sharpen

 There are many ways to sharpen a photo.

The most popular one in Gimp is unsharp mask.

There is also High pass.

Another way is to duplicate your image and desaturate the duplicate. Then go to the colour menu, down  to tone mapping then to Retinex. Apply it using the default settings. Once you have done that set the layer mode to overlay. You may want to change the opacity of that layer (the duplicated one) to taste. This usually brings put more detail in some photos

In GMIC there are many under details. It really depends what you want to sharpen. too. I go for Sharpen [gradient] as I do a lot of smaller photos and it is kinder.

Despeckle and Wavelet decompose

 Despeckle should be one of the first things you do if you have lots of white or black spots. There is also 'Descreen' in Gmic if the spots seem to follow a pattern. I have even used it to remove a flyscreen.

You may also like to use the wavelet decompose to remove areas of noise etc. Be careful not to remove your photo details though. I very rarely use this but some people swear by it.

Burn/Dodge tool

 This tool is good for black and white photos especially. As long as there is some detail in the photo. Burn will usually bring faded black lines out. Making them darker. You can do this with a brush or there is also a burn layer mode (where you would duplicate your layer and use the layer mode) Dodge can work wonders if a dark photo has detail in it. Not a fully black photo but a dark photo. It does the opposite of burn. Lightening areas. You can also use this as a brush or layer mode.

If only parts of the photo need burn or dodge. Bring your view up and work on that area

Levels & Curves

Levels are good for changing colours, bringing out the best colours in  photos. getting your white balance correct most times. You have a colour picker for white grey and black to do this. There are five channels in levels.

Value - take the slider to the left and it will lighten your whole photo. To the  right and it will darken it.

Red - Take the slider to the left and you will enhance the red in the photo. To the right and it will remove some red and replace it with green.

Green - Take the slider to the left and you will enhance the green. To the right and you will get magenta/dark pink.

Blue - Take the slider to the left and you will enhance the blue. To the right and yellow will be the dominant colour.

and Alpha

Curves also have five channels.

Changing to the different channels and moving the line up or down you will see a change.

The main one used in restoration is Value where you can lighten or darken a photo.