Hey! Lead designer for the game here! I'd call Retroshooti the "anti-Gradius" in that we were purposedly trying to break the rules of that kind of shmup gameplay design; we did get some ideas from Parodius though, and our favorite shmups like Terra Cresta, Star Soldier, Xevious, Recca, etc. Even then, it was always the idea to subvert shmups rather than imitate them!