I had a problem with GEEETech i3 printer.
GT2560 board and my PC connect was OK!
LED was blinkinc but didn’t works stepper motors.
I could not solve the problem for a long time.
But here is the solution.
Download: Arduino 1.0.5-r2
Download: Marlin PI-3_Pro_B_3DTouch firmware
Open this with Aurdino
You will repair BAUDRATE value to 9600
Next......
You setting in Repetier Printer Settings baudrate also 9600
Upload the firmware.
And it works!!!!!!!