It’s an expression of excitement and passion towards a certain situation, it could also be used as a greeting like hello or goodbye.
Bro I just got a new car!! “For realz?” Yup! “Gyuhhh”
Bro did you hear that new song by what’s his name? It was fire!! “Gyuhhh”
Bro you bout to dip out? “Gyuhhh”
Bro I just got married to the love of my life!! Gyuhhh!!
Mannn God is so Good bro!! Gyuhhh!!