Gus is often used as a short form of names such as:
- August, derived from Augustus meaning “great”, “venerable”, “majestic”;
- Gustavo (Spanish, Portuguese, Italian) and Gustav (Swedish, Norwegian, Danish, German) both deriving from Slavic Gostislav meaning “guest glory” from Proto-Slavic *gȏstь (guest) and slava (glory);
- Angus, the anglicized form of Scottish Aonghus either meaning “one choice” or “one strength, vigor, force”; the first element of the name comes from Old Irish óen (one) via Proto-Celtic *oinos (one) derived from PIE *óynos (one; single), while the second element gus meaning “strength, vigor” from Proto-Celtic *gusti- derived from PIE *ǵews- (to choose; to taste). In irish mythology, Aengus is the god of love, youth, and poetic inspiration.
- It’s also a Greek diminutive of Constantine which seems to be used primarily by Greek expatriates; it means “constant, steadfast”.
And there are likely other names from which the nickname Gus could derive from.