Based on the manga Salaryman Kintaro.
There`s also a game with the same name.
And a live-action series.

But this is the anime, by Emotion (Bandai Visual)

"Its about Kintaro, former leader of a widely feared and respected biker gang, who decides that its time tostraighten outand become asalaryman` since his wife died and he has to take care of his young son."

Sounds similar to GTO, but it isnt.
Well, it is a little similar, but its original.
I`d say its more like a modern day anime version of "Lone wolf and cub" then it is a GTO copy.