dude two words JO-JO! you thought it was another random comment,BUT IT WAS ME, DIO! edit : if he loves gate then maybe log horizon?
If he loves Gate, then he'll probably love Overlord; that is, if he can get past the first 2 episodes which are kinda slow. Gate also has a manga as well as the light novel of course. Both add to the overall story as the anime is forced to speed through certain plots. He'll probably also enjoy black lagoon and Jorgmungand. Of course, you can also never go wrong with one punch man and attack on titan.
Black bullet Outbreak company Monster musume Prison school (this you might've watched but it's 11/10 what you looking for if you've not) XD Drifters Hellsing Keijo Shokugeki no soma Assassin high school Shinmai Manu no testament (like high school of dxd) Some arn't what your looking for but good to watch ^_^
Here's another list: Serial experiment lain Monogatari series Blood+ Blood C Ghibli films Films directed by Makoto Shinkai Wolf children Bakuman Hikaru no go Chihaya furu Monster Yojouhan shinwa taike(haven't watch, but thinks it's a hidden gem) Ping pong animation(haven't watched it but heard it good?) Rainbow(manly bros/friendship prison stuff) Saiki kusuo no psi nan(comedy) Barakamon Natsume yujinchou Nodame contabile xxxholic And and uhhhh uhhhh someone help! cant rmb where the hidden gems!
Ai Yori Aoshi Mahou Sensei Negima! Isekai no Seikishi Monogatari (one of tenchi side story, I think) If he into mecha after the last title, then: Macross Code Geass Suisei no Gargantia Sidonia no Kishi Robotic; Notes
Ehm how about youkai apartment a bit slice of life but full of humourous stuff about supernatural Knight and magic If an old one and have ton of episodes gintama I rarely watch ecchi anime because most of their comedy is if not cliche one it will be only around perverted things the only one i think is really good is konosuba
Rurounin Kenshi Gantz Cowboy Bebop Trigun Code Geass Gintama MM Suzumiya Haruhi Kyoukai No Kanata Durarara Chi sweet home
K-On Gundam (can't really remember which one that been dubbed, except maybe Wing, SEED, OO, 08th MS Team, Char's Counterattack & Stardust Memory) Saber Marionette All Purpose Cat Girl Nuku-Nuku Love Hina Negi Magi JoJo's Bizarre Adventure Black Lagoon SDF Macross (the mashed-up dub ver would be Robotech) Gurren Laggan
3x3 Eyes El Hazard (One earlier Isekai animes) Dual! Parallel Urusei Yatsura Jubei-chan Conan(maybe)...
Do you mean Darker than Black, by any chance? I know Darker than Black - I remember watching it with Black Cat long ago - but I've never heard of Blacker than Black.