Extension cords won't break GFCI protection.

Yes, the GFCI still protects downstream devices through an extension cord.

The whole point of a GFCI is that it measures the current going out versus coming back. If there’s a leak—say the extension cord gets nicked and someone touches it—the GFCI trips. The cord doesn’t change how the GFCI works. So plugging a properly-rated extension cord into a GFCI outlet is fine, and anything plugged into that cord is still protected.

That said, there are two practical gotchas. First, very long extension cords (100+ feet) can have enough capacitance to cause nuisance trips on some GFCIs, especially with certain tools. Second, extension cords are temporary by design. If you’re running a cord under a rug or through