"Dad gummit" is a very simple thing to explain.
First, you use reverse dyslexia for the first two letters in each term.
"Gud dammit".
Then one simply fixes the first term by changing one vowel.
"God dammit".
And there you have it.
Dad-gummit this should not be such a trivial meaning! It's original as the idea of religion. It simply means "I sure as hell did not want "this" immediate outcome." Shit! I mean, dad-gummit!