The G label probably means 'earth'. And 'FG' is the 0V for isolated supply (not earthed). This is just a 'maybe'.
Get a multimeter in DC voltage reading mode.... disconnect everything from the power supply except for AC mains cable....turn on the supply....and measure the DC voltage between 'G' and the 5V terminal of the supply.
Then measure the voltage between 'FG' and the 5V terminal.
What values do you get for the measurements?
Later... if your AC cable has three pins when you observe it after disconnecting the mains plug from the wall socket...... you can use the multimeter to carry out a continuity test between 'G' terminal and the earth pin. And do the same for the 'FG' terminal and the earth pin.
Also ..... read post #3 if you haven't done it yet.
The issue is due to no user guide or tech manual for this power supply. Lack of details.
One more note or observation ...... the AC mains connection to that power supply looks ridiculous --- in that the AC mains wires are just connected to the terminal step on the front panel. Doesn't look like a good idea in terms of safety. I know some power supplies sold in shops are like that too, which is also ridiculous.