"GA", General American is the standard pronunciation and it's being spoken in the USA and other countries.
It is divided into four categories;
Consonants[ˈkaːnsᵊnənts]
Vowels[ˈvaʊəlz]
Stress[stres]
Intonation[ˌɪnəˈneɪʃᵊn]
Many a Niɡerian does find it interestinɡ but difficult to know as it's not taught in the university or in schools.
Furthermore, it's different from Nigerian standard English as there are lots of sounds not in Nigerian standard English such as [h] [ʰ] [ŋ] [θ] [ð] [ʒ] [ɻ] [ɝ][ɚ]