It comes in different formats and sizes

This should be clear already. A GDD it's not necessarily a text file written in a word processor. There are many types of software that can be used to write a GDD, which we're going to explore in the next paragraph. Different games of different sizes require different types of GDD.

The entire Office suite or Google Drive suite is usually required (Excel, PowerPoint, and Word, and their equivalents), but Word and PowerPoint can be replaced by a Wikia online, for example. Some games can be documented entirely on a blueprint such as document, a huge canvas where a mix of text and images can be arranged to convey the game design.

There are no fixed rules on this. You can even invent your own format if you feel like it. The only rule is to use the best tool for what you need to write.

The good news about all this variety is that most of the time, the Office/Google Drive suite would be more than enough, especially if you're just starting out. Learn and master as many tools as you can, and you will be able to tell which ones suit your requirements best.