That’s right. No extra data, no changes to the image. All the pixels and colours are exactly the same. Any data you want inside a GIF without any trace. But how? I’m glad you asked.
During highschool I was interested in steganography, the study of hiding messages in plain sight. I had just finished a simulation project that encoded GIFs, so I was still in the headspace of the GIF codec. I began tinkering with the different elements and found an area I wanted to manipulate. Here’s how I encoded 127 bytes into a GIF without changing the file size.