The answer to 'why is my GFCI outlet dimming lights' is that a GFCI (ground fault circuit interrupter) outlet can cause lights to dim due to the way it works. A GFCI is designed to quickly shut off power if it detects an imbalance in the electrical current, which can happen if a device is drawing too much power or there is a ground fault. This sudden change in power can cause connected lights to dim momentarily.
Some common causes for GFCI outlets dimming lights include plugging in high-wattage devices, faulty GFCI outlets, or issues with the home's electrical wiring. If the dimming is intermittent or only happens with certain devices, it's usually not a serious problem and can often be resolved by adjusting the load on the GFCI circuit. However, if the dimming is constant or accompanied by other electrical issues, it's best to have a professional electrician inspect the outlet and wiring.