I have a Grove Base Shield and it seems that it does not work with Arduino Mega and it works for Arduino Uno (it says it is compatible to Arduino Mega)

Products & Technology Shields
4

Hi Jian


You use the software serial for the grove gps. Not all pins on the Mega and Mega 2560 support change interrupts, so only the following can be used for RX: 10, 11, 12, 13, 14, 15, 50, 51, 52, 53, A8 (62), A9 (63), A10 (64), A11 (65), A12 (66), A13 (67), A14 (68), A15 (69). All of the signals are not routed to base shield. But the base shield supports analog/digital/uart/i2c as well.

https://www.arduino.cc/en/Reference/SoftwareSerial

As work around solution, You can modify the code to use the hardware UART directly.

Thanks

best rgds
Bill