There is a sound グパ in this manga (a plant tries to eat someone and makes グパ sound), but I can't find it in dictionaries and understand what it means and how it can be translated. What does グパ mean? Is it just something like "A-a-a"?
This is a mimetic word that describes the wide opening of a mouth or "a mouth-like thing". Sometimes it also describes how a covering/husk is cracked, like a watermelon or a human head cracked by an axe.
The better-known variant is unvoiced くぱぁ, which was popularized in the last two decades or so, and clearly has a sexual connotation (google it yourself; NSFW). Voicing of く makes it sound "rough", and it also avoids the sexual connotation.
I'm sorry, I don't know how this is translated into English...