When people talk about emulating retro games, the term "game" and "ROM" are often used interchangeably.
Because your GPi runs either RetroPie or RecalBox under the hood, what you're really looking for are ROMs to load into the emulators for that specific system.
What is a game ROM?
A game ROM is a single file that contains an entire set of game contents from the original game. The original console's cartridge contained a ROM, or "read-only memory", chip that stored the game in its entirety. When people talk about emulating retro games, the term "game" and "ROM" are often used interchangeably.
Types of ROMs
Game ROM files are generally universal for either system and are available in several types: copyrighted, abandonware, public domain, and homebrew.
Where to get ROMs
If you're looking for something like Super Mario Bros 3 or Super Mario Kart, that would fall under the "copyrighted" category. While you can find ROMs on any torrent site, you shouldn't download any copyrighted titles as this may be illegal.
In addition to torrents, you can find public domain ROMs on the MAME website and homebrew ROMs from PDRoms. Check out our complete guide to RetroPie ROMs to learn how to find and download all types of game ROMs!
Once you've found and downloaded a ROM or ROM pack, each file will be either in a .zip archive or a file with a system-specific extension such as .nes. For example, you might have Polybius.zip -- in this case, unzip it to reveal Polybius.nes. This is obviously an NES game.
Unzip each file
First, unzip each file to reveal the system ROM file.
Install each ROM
You'll need to transfer the ROMs to the GPi so that RetroPie can use them. There are several methods for transferring ROMs to the GPi: