Not sure if all of these are considered "hidden gems" but they seem not as popular. I enjoyed them nonetheless.
http://www.mangago.me/read-manga/the_best_wolf_in_my_life/
http://www.mangago.me/read-manga/i_like_you/ http://www.mangago.me/read-manga/suzume_favorite/ (and others from this author)
http://www.mangago.me/read-manga/misshitsu_no_hisoka_na_hoshi/
http://www.mangago.me/read-manga/yasashii_koi_ga_sodatsu_made/
http://www.mangago.me/read-manga/perfect_xxx/
Not really underrated, but not a lot of people seem to know them so try these out I guess. I have more lol
http://www.mangago.me/read-manga/vampire_juuji_kai_fallen_vampire/
http://www.mangago.me/read-manga/chrono_crusade/
http://www.mangago.me/read-manga/psyren/
http://www.mangago.me/read-manga/yandere_kanojo/
bored :( list some underrated hidden gems please