Whole House GFCI Trips In Cold Weather

Condensation

Some part of the circuit - a receptacle, switch, or junction of some sort - has a very minor ground fault. It is OK when warm and dry. When the air cools at night down to the dew point, water condenses. It bridges the ground fault and enough current flows through to trip the GFCI.

With one quick search, I found a current dew point in Bangkok of 60 F. It will vary a bit by day, time and location, but that isn't far off from the 68 F you noted.

The only solution is to check every single junction in the problem circuit. In particular look for any bare wire (except grounds), loose wire nuts or signs of corrosion. You can rule out, initially, any light fixtures, since you have the problem even when they are off. However, wires often go through those boxes on the way to other parts of the circuit, so the problem may be there too. You may have multiple sources of the problem, though since all your other circuits are OK, there is a good chance there is just one problem area.