GFCI part.. what is it?

I can't figure this one out the one with the spring, long story short the GFCI would not reset or if i did reset it and press the test button the GFCI would buzz and ended up throwing a spark inside the switch housing but I don't see any damage so I'm not sure if that part was laying on the resistor, GFCI test post, or the ground.

It's the plunger out of that little solenoid (little square silver metal thing with the hole in the middle). It's supposed to mechanically latch the on position, then when a fault occurs the solenoid pulls in the plunger, releasing the contacts. It's most likely just been dislodged by getting tossed around with the extension cable

I should also mention that if for any reason any ground fault interrupter fails to operate as expected, it should be replaced to ensure safety, even if you repair it and it appears to be working.