Why does my Pokémon Gold cartridge seem to forget saved gameplay?

I recently bought a copy of Pokémon gold version at a car boot sale. I played it for a while but then I had to stop, so I saved and then turned the game boy off, and took the game out. The next day when I put it back in my Game Boy and switched it on, all my save data was gone and the only options were New Game and Cancel.
I checked online to see if it was fake, but it looked the same as all the real ones. I also compared it to my Pokémon red which I know is genuine, and all the details of the cartridge were the same (except for colour and sticker of course) here are some pictures of the comparison between the two games
Does anyone know whether this copy of Gold IS fake? How can I tell? Or is it just broken?
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Do Gameboy games save on the cartridge?
Many game boy games save the files on the cartridge. The cartridge is able to save games because of the battery inside every cartridge. If the battery inside the cartridge dies, the gameboy board will not have the ability to save or overwrite a saved file. This is something that upset quite a few gamers.Why does my Gameboy game not save?
So Why Does A Game Boy Color Not Save? The circuit board of the cartridge has a memory card attached to it, and after a year or so, the memory card battery goes bad and does not save the games anymore. So, in order to fix the problem, you will have to replace it with a new battery or possibly a new game cartridge.Why is India not interested in taking back PoK?
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Oleksandr Pidvalnyi, David McBee, Pixabay, David McBee