What does GUU stand for?

Grundarfjordur - Iceland - Airport symbol (OAG)

Guanine Uracil Uracil - A triplet of bases coding for the amino acid, valine (EES)

Gulfstream USA Inc., North Miami, FL - Manufacturer code for watercraft hull identification number (USCG)

Gulu - Uganda - Airport symbol (AD)

Yanomamo - Language code (ETLW)