Where to attach the c-wire inside Goodman GMP100-4 Furnace
That black box in the top left is the 24vac transformer that powers the circuits of the furnace.  The blue wire that appears to have a brown ground wire piggy-backing onto it is what the C wire needs to connect to.  Sometimes there is a connector exposed on the circuit board for a C-wire connection, but not always.

If there's no other place to connect the C-wire, under that ground wire screw would be once place to connect it.

Use the blue wire in your thermostat wiring for the C-wire - that's the common color for it.  Don't use the black.

On second look at the pictures...  The leftmost wire nut in the bottom of the pictures has a thick bluish wire that goes up to a ground screw and a small red wire that goes somewhere else.  You could tie the C-wire in that connection rather than trying to put it under a ground screw yourself.