What even are Gigapixels photographs?
Gigapixel is basically a panorama on steroids. They are usually merged together from hundreds of photographs to create a huge ending photo. To get the best results it is necessary to put your camera on a tripod and do some preplanning. 
Usually gigapixels are taken with a much longer lenses like some telephotos at around 200mm to get the most details. In some extreme situations people also use much longer lenses, even 400mm or more. But those gigapixels are usually done with a special hardware. An automated system you mount your camera on to and it does all the work for you.
If you want to try and create something gigantic you don't need any special hardware but you will need some software to merge all this photographs together. I still use ancient and discontinued ICE (Image Composite Editor) by Microsoft that works great but it is long gone from their website to download. Many people say good things about PTGui and Hugin. I might try those two out just to see how they work out.
When you want to take a gigapixel panorama you preplan how your photo will look and set your tripod on a sturdy surface. It is very important that your camera pans straight in a line so be careful that it is in level. Then you take multiple photographs in direction from left to right and repeat many rows from top to bottom.
Check out some of my own gigapixel panoramas below.
This is an example from my 1.12GP (44971x25000px) or 1120MP taken from 136 24MP photographs, 8 rows of 17 photographs.
Final result: "Magical winter sunset with mountain Triglav"
Check out 100% zoom previews
"Gigapixel photograph of mountain Triglav, Slovenia"
100% previews from my first gigapixel that was "just" 350MP (24726x14161px) big
I have many more I will upload here in the future.