The Sega's both systems had almost exactly the same hardware with the only difference being that one was mobile and the other handheld. That enabled game developers to easily transfer games from one system to another.
Specs:

Master System Bits (CPU) (8): 8 Bits (Gx): 8 CPU: Z80 MHz: 3.6 Graphics: 240 x 226 Colors Available: 256 Colors on screen: 52 Sprites: 16 Sprite size: 8x8 Audio: Mono RAM: 64Kbits    Game Gear Main Processor: Z-80 (8-bit) Processor Speed: 3.58 MHz Resolution: 160 x 146 Colors Available: 4,096 Colors on screen: 32 Maximum Sprites: 64 Sprite Size: 8x8 Screen Size: 3.2 Inches Audio: 4-lyr RAM: 24K Sega Master System Sega Game Gear

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