Right,
For a start, let's stop calling this a GFCI, that term is only relevant in the US.
What we have outside of the US is a Residual Current Device (RCD), it is meant to trip with a current imbalance of 30mA, with a maximum trip time of 300 mS (milliseconds).
Unless you have the relevant test equipment to test this equipment, you are wasting your time trying to fault-find this, get someone that has this equipment to test it, you will not be able to test this installation with a multi-meter.
You could possibly have a faulty RCD or one that has not been installed correctly feeding current back into a Earth busbar or a Neutral busbar.
Does this happen at any particular time of the day or week?
