I'm sharing here an answer I gave to a concerned friend on Facebook. I hope this will help others who have been wondering too. Here we go:
Shalom! AnaRina here. Thank you so much for your comment. And thank you for showing concern about this! It means you take G-d's name and how It is represented serious and are concerned about respect for His name.
This is the very reason (out of respect for His name) that we as Jews (I am an Orthodox Jew) write His name like this. As you know, Hebrew is the original language of the Bible. When the Jewish people received the Torah on Mount Sinai (which we will celebrate on the Festival of Shavuot) and even the way that after 3500 years we still read it today, it was given to us by G-d in Hebrew. The Hebrew language, (except for certain instances regarding the letters aleph and ayin), has no vowels. Even our newspapers and books that are written in modern Hebrew, also don’t have vowels.
So the first reason is that we keep to the original way that it was written in the Bible without vowels.
The second reason is that the holiest and ineffable name of G-d, which is spelled with four letters (the Yud then the Hey then the Vav and then again the Hey) or the tetragrammaton as we refer to it, is not allowed to be pronounced and also not written in daily writings as it is too holy. This name of G-d was only pronounced once a year on Yom Kippur and it represents G-d in His deep manifold ways that no man on earth can claim to know in full. Because this extremely Holy name represents G-d, we ascribe to it the highest honor. And therefore even in English, which the simple letters G, O, and D have no connection to any of His names as He revealed to us in Hebrew in the Bible, we still in reverence recognize that when we speak of Him or refer to Him, we write it differently.
A third reason is that the word Gad, the name of one of Jacob's sons, in its root means luck. There was also an ancient god named Gad. You can hear in the pronunciation that that is very close to the way the word "god" is pronounced.
So to make EXTRA sure we don't confuse the Most High (to Whom no-one can be compared and to Whom no-one is equal) with any other entity, we write it the way we do.
Hope that helps!