MPa stands for megapascal. The G stands for gauge which means the pressure was read from a guage.
PSI stands for pounds/square inch. The G stands for the same as above.
When pressure is read from a guage, its actually measuring the difference of the pressure in the pipe and the pressure of the atmosphere. As an example, when the pressure inside the pipe is 1 atmosphere, the guage will be reading 0, because the pressure inside the pipe is exactly the same as the pressure outside the pipe.
If you need Absolute pressure, you need to add 1 atmosphere to the reading of the guage. But you do not need to do anything, because in your case you are not converting to absolute pressure, you are converting from one guage measurement to another guage measurement.