Assalaamu alaykum. Is the expression 'Goddammit' a very great sin, Islamically?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.

The term that you asked about is a form of cursing. There is no harm in cursing something that the Sharia deems permissible to curse. The Prophet, sallallahu ‘alayhi wa sallam, cursed the scorpion; it was narrated that ‘Aa'ishah, may Allah be pleased with her, said, "The Prophet, sallallahu ‘alayhi wa sallam, was stung by a scorpion while he was praying, so he said, 'May Allah curse the scorpion, for it does not spare the one who is praying and the one who is not. Kill it  regardless of whether you are outside the sacred precincts of Makkah or inside them.'" [Ibn Maajah] As-Suyooti said, "It is deduced (from this hadeeth) that it is permissible to curse harmful things..." [Sharh Sunan Ibn Maajah]

However, if the thing does not deserve to be cursed according to the Sharia, then it is dispraised to curse it, even if the thing that is cursed is an animal or an inanimate object. Imaam An-Nawawi said, "The same applies to cursing all animals and inanimate objects; it is all dispraised ... Abu Ja‘far An-Nahhaas narrated that a scholar said, 'If the person curses what does not deserve to be cursed, then he should hasten to say, 'unless it does not deserve (to be cursed).'" [Al-Athkaar]

The scholars explicitly stated that the person bears a sin for cursing what does not deserve to be cursed and that such cursing is prohibited. Imaam Ibn ‘Abd Al-Barr said, "Whoever curses what does not deserves to be cursed bears a sin for it..." [At-Tamheed]

Mataalib Uli An-Nuha (a Hanbali book) reads:

"It is prohibited to curse the animal; it has been reported on the authority of ‘Umar that the Prophet, sallallahu ‘alayhi wa sallam, was traveling once and a woman cursed a she-camel. So he said, 'Unload it and set it free, for it is cursed.' ‘Imraan (the narrator) said, 'As if I still see it (the she-camel) walking among the people and no one got in its way.' [Ahmad and Muslim] Another version of the hadeeth, narrated by Abu Barzah, has the following addition, 'A cursed she-camel does not proceed with us.' [Ahmad and Muslim] Abu Ad-Dardaa' reported that the Prophet, sallallahu ‘alayhi wa sallam, said, 'Those given to cursing will not be witnesses or intercessors on the Day of Judgment.' [Muslim] Ibn ‘Abbaas narrated that a man cursed the wind for blowing away his garment in the presence of the Prophet, sallallahu ‘alayhi wa sallam, so the Prophet said, 'Do not curse the wind, for it is doing what it is ordered to do, and whoever curses something undeservedly, then the curse returns upon him.' [Abu Daawood]" 

Shaykh Bakr Abu Zayd said:

"In Islam, one of the financial penalties for cursing is that if a person curses a riding animal, it must be released and set free. The Islamic Sharia strictly sought eliminating cursing what does not deserve to be cursed. The Prophet, sallallahu ‘alayhi wa sallam, forbade cursing the rooster and the flea. The wise Muslim should guard his tongue and avoid cursing or returning a curse. He must abide by the Sharia provisions in this regard and not curse except people or things that deserve to be cursed based on textual proof from the Quran or the Sunnah..." [Mu‘jam Al-Manaahi Al-Lafthiyyah]

We did not find any statement of the scholars that rules cursing to be a grave sin, except for cursing a fellow Muslim. At-Tabaraani narrated with a good chain of narration that Salamah ibn Al-Akwa‘ said, "When we saw a man curse his brother, we used to think of it as a kind of major sin."

Allah knows best.