I had a decent amount of fun with it.
The visuals are definitely the best thing about this. You put a lot of detail into making the environment, and it shows. The colors being scattered red, blue and yellow everywhere is a tiny bit distracting on the eyes and doesn't look as cohesive regarding giving a stage a color scheme, but I think that also helps nail down the idea that it's a harbor with various different types of crates everywhere, as if they all came from different places.
The level design is actually best in 2D I thought. The areas of open movement were rather nice, but I'm gonna be honest about the engine: GDK really made it less fun than it should've been. The clunky jumping making me misfire my jumps/homing attacks a lot, rocketing off of downhill areas instead of sticking to them, and the clunky turning just kind of brought the feel down by a large margin, and only emphasized the areas that have almost unfair 'insta-death' spots. The level layouts themselves though were hardly bad though, as I'd imagine putting that in the Generations engine would probably play like a dream. I guess GDK doesn't suit the Unleashed/Gens level design too much.
But yeah, fantastic job either way! You did this all on your own, and that's an achievement if I ever saw one. Kudos!