To further explain, the character used here is from the "alphabet" japanese uses exclusively for onomatopoeias and foreign words, katakana. Its just for sounds, so it has no inherent meaning other than a "go" sound repeatedly, and as said its used to convey a menacing atmosphere.
This is also how オラ means no more than an "ORA" sound but 無駄 (muda) has an actual meaning associated with it, "useless" in this case.

See Also:

• gitaigo
• japanese_text
• jojo's_bizarre_adventure
• sound effects