While far from the worst SHMUP I've played, this offers nothing new in terms of gameplay mechanics or, anything really. The best thing I can say is that the boss designs are just fine.
Also the final boss is not easy, even on 'Easy' mode. So I spent maybe 45mins going through most of the game then spent 2+ hours on the final stage, which is very short but I died a whole lot. Not necessarily a bad thing, but poor pacing, and would have liked an easy mode that's actually close to a 'light' challenge.
D-pad controls feel stiff, expect delay, and not ideal for the fast-twitch gameplay required. You will experience cheap deaths.
I'd rate this a '6' if it were still a budget title, something I picked up used for just a few bucks, but at current prices this game is painfully mediocre. I'd rather play any other SHMUP I own than play through Gekioh again.