Gigantis is a Manga/Manhwa/Manhua in english language, GORE series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.nl fastest updating comic site. The Summary is

On that day, humanity was turned into “prey”―Gen is a high school student living on the island of Tsushima, a door to mainland Japan. Along with his sickly sister, Misaki, and his heartwarming mother, Gen was living a peaceful life, all until an “asymmetrical monster” appears on the island. All hell breaks loose on the island of Tsushima. These monsters devour everything from plants to animals, to humans, and mutate together. Will humanity stand a chance against these mutant creatures? Is there any hope?