Yes, pin 27 is GND for JAMMA pinout, but is used for button 6 on these multi boards (known as CHAMMA - Chinese version of JAMMA). Just make sure your grounds are connected to both sides of the edge connector (so, pins A, B and f). Not having any buttons/sticks connected won't matter, BTW. The Game Elf will just read all the controls as being 'not pressed'.
Oh, one more thing. Make sure you have the edge connector plugged in the right way around! If you plug it in upside down, you'll put voltages into the board where they shouldn't be, and probably fry it.