I was recently inspired by this post at Swistle Baby Names to do a post on the nickname "Gus". in the post, the letter writer states that she is considering the name August, but only because they like the nickname Gus. As I pointed out in the comments, there are many other ways to come to Gus! This is one of those nicknames that was used to be a "funny" name when I was a child. My dad would throw it out there when he was pretending to forget one of our names, or we would use it to name a frog we caught. Now it is the exact kind of nickname that seems just right - a bit of vintage charm with cute familiarity, like Hank and Fred (fifteen years ago, Jack would have fit this category). On to the name!
August/Augustus/Augustine - All of these are derived from Latin meaning "great, magnificent". The different versions carry different feels though. August can come off as a "quirky" word name due to its use as a month. Augustus feels regal and Roman, while Augustine has a religious tone. These also offer the adorable nickname "Augie".
Aengus/Angus - Technically speaking, these two names are pronounced similarly, though I am sure you will find plenty of people will pronounce the the same or switch the pronunciations. Aengus (with a long "a", rhymes with bay) is the original Gaelic name, which means "one choice". Angus is the more anglicized version, and is pronounced Ann-gus.
Argus - I have discussed this name before in the Harry Potter "A" post. It is a Greek name meaning vigilant guardian.
Fergus/Ferguson - This is another Scottish/Irish name, meanig "supreme man, highest choice". This may be one of my favorite options! It reminds me of other names that I like but not quite enough to use, like Vernon and Fern.
Gustavus - Pronounced Gus-Tay-vus, this is a "last name as first name" related to as Scandinavian name meaning "royal staff". It is also the name of a University.
Lugus - Pronounced Lou-gus, this is a Welsh name meaning "shine". It is very similar to the more common Lucas, but with a little twist to make it unique.
Magus - This Latin name related to the word "magic", meaning sorcerer. It sounds similar to the more feminine Maggie. I would be a bit worried about the phonetic nearness to Maggots however.
In addition to the above names which make the nickname Gus a no brainer, there are also names that would get to it a little less directly, for example, Auguste (aw-goost), Gustav (Goo-stawf) are the more European pronunciations of some of the names above. Other options would be Magnus (Latin for "great", as in magnificent), Agathius (Greek meaning good, honorable), Eligius (french/Latin for "chosen"), Eugenius (Greek for "well born"), Giorgius (a variant of George), or Sergius (a variant of Sergio)/
Of course, if none of these strike a chord, just use Gus!