"God damn" or "goddamn" or "goddamnit" are all considered not just curse words but depending on the person and how religious they are or how religious they were raised they're top tier "no no" words. Taking the Lord's name in vain is in the Ten Commandments and all that. You can include exclamation of "Jesus Christ!" or obviously variations of "Jesus Fucking Christ!" just "Jesus!" "God!" in that list too. Adding fucking is particularly bad. Just exclaiming the names in frustration or anger or whatever is enough for a face slap in a lot of religious families when I was growing up. I was never slapped, but my grandparents used to pretty openly tell my parents that my brother and I were definitely going to Hell because we said "God, Mom! Leave me alone!" too much or whatever.

"Hell" and "damn" are both curse words but on the very low end. A lot of the Christians who would cover their mouths if someone said "goddamnit!" around them would be ok with a "damnit!" or just a "get the hell out of here!" although it should still be reserved for only situations of anger, surprise, etc. and you should still feel bad. Also still generally forbidden for children to say them outside of the context of speaking about Hell the place or God damning someone (Biblical loopholes. Jesus understands the space and how important it is!)