Hyperkin Lab have launched the Smart Boy, an add-on for Android phones that will enable users to play Gameboy & Gameboy Color cartridges on their phone. The Smart Boy will have an open source serial app and firmware. Hyperkin will also be offering a royality percentage for developers who help improve its app and firmware.
There is an Atmel 90USB646 chip in the device and is NTSC/PAL cart compatible.The device will also require Android 4.4.4 KitKat and above, with USB OTG support. Also, the kit is usable with 5-6inch devices. The Smart Boy is expected to ship by December 2016 and its pricing hasn't been announced yet.
Older games have been ported to Android in the past, with Counter Strike 1.6 being a good example. That said, playing Gameboy games would be a different thing altogether.