I found the word グギイイ, while reading a comic and I'm having trouble figuring out its meaning. The only thing I can think of is that "グギ" could be a sfx for "popping veins", seeing as the character is getting angry at his friends for being lazy; but the word is not listed on http://thejadednetwork.com/sfx/ and it's written inside a speech bubble, making it look like the character is shouting it. This is my first post, so I hope I'm not violating any rules. Thank you very much for whatever help you'll provide!
Onomatopoeia are infamously difficult to translate, since there are around 3 times as many of them in Japanese as in English. They can also vary from region to region in Japan, and even from person to person.
In general, when the onomatopoeia uses voiced consonants (with dakuten like ぐぎ) it is denoting something forceful. Remember that voicing a consonant means you have to vibrate your larynx, and it is this extra vibration which is said to provoke an impression of force or intensity in the listener. Some typical examples:
ころころ something small rolling vs ごろごろ thunder, purring
さらさら silky vs ざらざら rough
So I'd say from the context it is likely to mean something forceful, something gutteral, something strong or powerful, something which is intended to convey intensity.