The term manga is a general term in Japanese; all comics of all genres are manga. When it was coined, however, "manga" meant "irresponsible pictures", similar to how "comics" in English originally referred to humorous newspaper comics. When comic artists in America wished to create serious works with darker themes, they began to call their works "graphic novels" and "sequential art". The term "gekiga", meaning "dramatic comics", was coined for the same reason.
In the 1950s, 60s, and 70s, Gekiga were Japan's alternative comics, with a visual style very heavily influenced by the realism of American comics. Often the work of a single creator, or a creative team of artist and writer, they tended to be gritty, intense works with more detail per page than manga published in magazines. Originally, many gekiga were published through "kashibon", rented books at commercial lending libraries, because the lending libraries accepted more controversial subject matter and allowed for more artistic freedom than mainstream publishers would grant at the time. Gekiga were also published in yakuza-run "kasutori zasshi" (moonshine magazines), containing pulp fiction with depictions of transgressive sexuality and violence.
Gekiga were extremely popular among rebellious teenagers and young adults in the 1960s and 70s, similar to the popularity of rock and roll in the same generations in America. However, by the late 70s, the influence of gekiga on mainstream manga had dulled much of its shock value, and the adoption of reader polls and exclusive contracts by manga magazines resulted in a market ill-suited to gekiga. While it remains as a genre, gekiga is no longer the future of underground comics in Japan.
Gekiga are difficult to find translated in English, since they are not in high demand. However, Sanpei Shirato's Legend of Kamui was among the first manga published in the United States.