It is rarely used literally, and is considered a lesser of vulgarities, though to the extremely religious it is considered highly offensive. In the Christian faith it is considered a violation of one of the ten commandments.
God damnit! I hate my fucking job!
God damnit! I have lost faith in the human race!
God damnit! Why didn't my mom have an abortion?
From it's originated state, it has become somewhat false in the English language, where many still belive it's just "God Damn It!" without spaces. God himself knows this is wrong.
Today it means something (1.) important, and/or (2.) something worth making a statement over. It can also mean that you are (3.) embarrased over something.
2. This will be something that people will talk about, goddamnit!