Where should I connect a C wire on my Goodman PGB030075-1 furnace?
Right part, wrong wire

While the terminal you have circled does attach to the transformer, it attaches to the primary side of the transformer (i.e. the AC mains side).  You need the secondary side common, which on your system is the blue wire coming off the transformer (if there are multiple blue wires leading to that terminal, you can tap any of them).