Normally you open GIF files like other images in an image editor and also web browsers support them natively. However, if you want to know all the details of the GIF image file format other tools have to be used.
Now if you want to see all the details of a GIF file, there’s a GIF grammar for Synalyze It! (macOS) and Hexinator (Linux and Windows). You can download the grammar from the Synalyze It! web site or directly within the application.
Wikipedia has a comprehensive article about GIF files so not all the details will be repeated here. If you want to learn more about GIF files the article What’s in A GIF – Bit by Byte is worth reading. Another great blog post is Inside the GIF file format, written by Joshua Davies.