The answer to 'why is my GFCI outlet dimming lights' is that a GFCI (ground fault circuit interrupter) outlet can sometimes cause lights to dim due to the way it functions. GFCIs are designed to quickly detect ground faults and interrupt the circuit to prevent electric shock, but this sensitivity can also affect the voltage supply to connected devices like lights.
Common causes of GFCI outlets dimming lights include: 1) the GFCI is old and needs to be replaced, 2) there is an issue with the electrical wiring or grounding, or 3) the GFCI is overloaded with too many devices. In most cases, the solution is to try resetting the GFCI, replace the outlet if it's old, or have an electrician inspect the wiring. If the problem persists, it's best to call a professional electrician to diagnose and resolve the issue.