Krool Games regularly develops new games for Nintendo's 25-year-old handheld.
McDonald's isn't the only company to pull off a retro-gaming-themed PR gambit today. Atari is also releasing a new game for the Atari 2600, an announcement that also served to drum up interest in its upcoming title Mr. Run and Jump. If Halliburton announces a new Sega Master System game about fracking in the next 24 hours, I'll know someone is messing with me.
I've just spent 20 minutes playing this and ragequit on the third level. It's entirely possible that it's much easier to control on an actual Gameboy, but for keyboard it's...challenging.
I think releasing a marketing game like this in .gba format is great, it's fairly accessible on non-GameBoy hardware and its more interesting than just a basic app/game for a company.