I'm totally puzzled as to what this means, and I couldn't find any helpful hints when I searched through multiple websites. Does anyone else have a clue?
It's a variation of
ギギギ, which originates from the manga
はだしのゲン (barefoot Gen).
It's onomatopoeia which refers to teeth grinding in anger, frustration or regret. It can also be used as a sort of "groan" symbolizing unbearable agony.
See also http://www.paradisearmy.com/doujin/pasok_gigigi.htm for more info.