The only reason that you would feed a low voltage light that originates from a GFCI is if the 120V/12V transformer were located in an area that required ground fault protection. A low voltage light (12V) won't trip a ground fault breaker (120V).
When using low voltage lights, the transformer MUST be rated for pool/spa applications (The transformer must have a grounded metal barrier between the primary and secondary windings).
For 120V swimming pool lighting, what's most important is where the junction box for the lights is located. For fountains, the box can be flush with the pool floor, but the box must include cord seals and must be filled with reenterable potting compound.
