Those messages indicate that although the serial port selected in GroundControl is alive, the data coming from that port is not the expected data. You can troubleshoot this in the Arduino program after you’ve uploaded the program to the Mega2560. After you see ‘done uploading’, open athe Serial Monitor from the Tools menu. Make sure the baud rate is 57600. You should see something like:
ready
ok
Grbl v1.00
<Idle,MPos:0.00,0.00,0.00,WPos:0.000,0.000,0.000>
[PE:0.00,0.00,127]
<Idle,MPos:0.00,0.00,0.00,WPos:0.000,0.000,0.000>
[PE:0.00,0.00,127]
<Idle,MPos:0.00,0.00,0.00,WPos:0.000,0.000,0.000>
[PE:0.00,0.00,127]
<Idle,MPos:0.00,0.00,0.00,WPos:0.000,0.000,0.000>
[PE:0.00,0.00,127]
<Idle,MPos:0.00,0.00,0.00,WPos:0.000,0.000,0.000>
That’s what GroundControl expects to see.