How to GFCI protect bathroom lighting circuit
Based on the description you've provided in your comment. You should be able to install a combination GFCI switch device in the first bathroom (electrically closest to the breaker box), which will provide GFCI protection to the light and the rest of the circuit. Install the GFCI switch combo as follows:


All grounding conductors left off for simplicity. Make sure all devices are properly grounded.


Connect the grounded "neutral" (white) conductor from the feeding cable, to the silver LINE terminal of the GFCI.
Connect the ungrounded "hot" (black) conductor from the feeding cable, to the brass LINE terminal of the GFCI.
Connect the grounded "neutral" (white) conductor from the cable going to the light, to the silver LOAD terminal of the GFCI.
Connect the switched (red) conductor from the cable going to the light, to one of the switch wires of the GFCI using a twist-on wire connector.
Using a twist-on wire connector, connect the other switch lead from the GFCI to the ungrounded "hot" (black) conductor from the cable leading to the light, and a short bit of black wire. 
Connect the other end of the short bit of black wire (from the previous step), to the brass LOAD terminal on the GFCI.
Appropriately connect all grounding conductors.