According to Sennin :
Thanks,
Do you think it would help to change the whole transformer? I hate the
prospect of digging up my malibu wiring and relaying it.

Unless the GFCI is faulty, having a GFCI trip because of a LV
lighting system means that something is definately wrong with the
transformer itself.

Even if the LV wiring was short circuited/grounded et. al., the GFCI
upstream of the transformer will _not_ trip. For a LV fault to
trip the GFCI, the transformer would have to have to be faulty too.

It's more likely that it's the outlet or high voltage side of the
transformer.

For the GFCI to trip, there must be a leakage current from the neutral
or hot to ground. If the transformer is okay, this can only happen on
the high voltage side of the transformer, or in the outlet it's connected to.

I'd _tend_ to suggest that the hot side of the transformer or the
transformer itself is getting damp, and a conductive path is present
to ground. Possibly through the transformer frame. Alternately,
the transformer has a coil-case short, and moisture is providing
case-ground conductivity.

Alternately, the outlet itself is damp/corroded or gunked up and
the transformer is providing the hot-ground path.

Disconnect the unit, bring it indoors, let it dry out thoroughly
and clean it. Especially crud around the plug, or where the
wires enter the transformer. If the transformer unit is easily
opennable (not a plastic molded case), open it up and clean it.

With the power _off_, open up the outlet, check for moisture,
corrosion or debris (particularly bug bits). Clean out and dry.
Make sure you reassemble the seals properly.

When you put the transformer back outside, try to shelter it from
weather, but don't seal it completely, because that will lead
to condensation.

Then see what happens. If it still trips, I think you need a new
transformer.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.