The website ニコニコ marks manga with sexual content with the letter H and with excessive depiction of violence with the letter G. If you really feel the need to see an example, this manga is marked with both. In the desktop version, there is an explanatory text beside them, reading 性的な描写 and 過激な暴力描写 respectively, but on the mobile version there is no such explanation, only the letters.
I understand why "H" is used for sexual content, but I don't know why they chose "G" for "excessive depiction of violence". 暴力 bōryoku "violence" starts with "B", 過激 kageki "excessive" with "K" and there is also no English word that I can think of. The only thing with "G" would be the 激 of 過激. This could be a shift "excessive => excessive violence", just like for the English words "explicit" and "graphic". I have only seen "G" as euphemism for ゴキブリ cockroach, and searches for "G"+暴力 or "G"+"過激" yield no relevant results.
This leads me to two related questions: Firstly, would a Japanese speaker recognize what "G" as a warning meant without the explanation? And secondly, why did niconico chose "G"? I am kind of hoping for an interesting story like behind the "H", but maybe it was just an arbitrary choice with not much thought behind it.