"Gus" Grissom via www.astronautsmemorial.org
While Gus can be a nickname for Gustav(e), Angus, and August, and variants of those names, Gus itself ranked #999 in 2016. It hadn't been in the top 1000 since 1978. Mingus, Argus/Argos, Fergus or Ferguson work just as well, and a rarely considered option is the Norman name Guiscard, which is cognate with the word "wizard." Should you prefer using Gus or Gussie on a girl, most often those nicknames are used for Augusta, Augustine, etc. However, Gustava and Gustavia are female variants of Gustav.
Real-life namesakes for Gus include NCAA basketball announcer Gus Johnson, poker pro Gus Hansen, astronaut Gus (Virgil) Grissom, American skiier Gus Kenworthy, and film maker Gus Van Sant. A few different celebrities have chosen this (some as a nickname for either August or Augustus) for their sons.
In media, Gus-Gus was the name of a mouse in Cinderella, a character in T. S. Eliot's Old Possum's Book of Practical Cats, and a character in the book The Fault in Our Stars by John Green. In TV world, Gus has been a character in "Psych," Netflix's "Love," Disney's "Recess," "Breaking Bad," "Road to Avonlea," "Queer as Folk," and "Mighty Med."
Should you prefer Augustus as a full name with Gus as the nickname, Augustus is Latin meaning "venerable," and was originally used as a title for religious leaders in ancient Rome who acted similarly to prophets or oracles. The transfer out of religious-only use was made when Caius Octavius won a battle and the name Augustus was given to him as an honor, and future emperors thought they would use it as well. It may have taken all the way until the 1500's until the name was used as a chosen (and only) given name. Augustine was much more common. This name is not popular in England, Wales, or Scotland, and has not been since the 1940's.
Gustav is German and either means "guest of glory," as a Germanization of Old Slavic Gostislav, or Old German "God staff," (though there is debate that it means "Goth staff"). Gustave and Gustav haven't ranked since 1934, but Gustavo is at #533. The list of namesakes is quite long, including Nobel prize winners, artists, musicians, writers, Olympic medalists, a king of Belgium, a Belgian Prime Minister, and a French president.