"Gamba, a town mouse with a brave, adventurous spirit decides to go on an adventure to discover the ocean. On his way, he meets a troubled child mouse, Chuta. He says his family and other mice have been killed by a clan of weasels living on a nearby island. Chuta asks a group of ship mice, the bravest of their kind, for help, but when they find out that the villain behind this event is the terrifying weasel-leader Noroi, they give up on helping Chuta. However seeing the hopeless Chuta, Gamba promises him to save the island, his family and the other mice. Gamba and his friends team up to go on their adventure to defeat the evil Noroi and his clan." (Scammed from here).
That is really a lovely song but seems pretty old school for modern animation. This song was sung during the opening and closing credits and had some nostalgic looking stop-action animation. Based on the audience of a half-dozen older ladies and NO kids, I surmised that this was based on an old animation.
I was right. I did some digging and discovered that there was indeed a previous movie called ガンバとカワウソの冒険 or Gamba to Kawauso no Bōken (Gamba and the River Otter Adventures).
Which was a sequel to a 26 episode animated tv series called ガンバの冒険 or Ganba no Bōken.
Which was based on a series of kids books.
Speaking of old animation, here is the making of the opening credits sequence.
That's just a taste, you can find a whole lot more of these here.
Here are a couple more stills from the movie.
カワウソ (Kawauso = River Otters from the prequel) but they were equally spooky. They were led by an albino weasel named NOROI who was damned creepy.
Gamba was voiced by Yuuki Kaji (Eren of Attack on Titan fame) and he was indeed spunky. His girlfriend was the gal who voiced Anna in Frozen, Sayaka Kanda.
Here's an interview:
By the way, the Executive Producer is none other than アヴィ・アラッド, better known to North American audiences as Avi Arad of X-men, Hulk and several other Marvel movies.