Using a modern handheld (like a Steam Deck or a modified Game Boy shell with a Raspberry Pi inside) to stream the PC game via Wi-Fi.
Since Watch Dogs is a modern, high-fidelity PC and console title, it cannot run natively on Game Boy hardware. However, in the world of "highly compressed" gaming and retro-emulation, people often look for unique ways to bridge that gap. Here is a blog post exploring this concept. watch-dogs-pc-game-download-highly-compressed-gameboy
If you want to play Watch Dogs with a handheld, retro vibe, here is the legitimate way to do it: Using a modern handheld (like a Steam Deck