Science fiction cartoon movie taking place in an apocalyptic kind of world

The Jules Verne novel, Off on a Comet published in 1877 contains all the elements you mention.

Remember a place with low gravity, still on earth. The world was apocalyptic; lots of lava, lots of ice and sea.

A comet passes so close to the earth that it peels off a small piece of Mediterranean coastline, air, water, people, goats and all, and pastes it onto itself. There was a volcano on the comet (!?) that helped them when the comet's orbit went rather far from the sun, and the sea froze.

Main characters were in blue uniforms, soldiers most probably. 2 main characters one of them thin and intelligent, the other one was a bit overweight and had a mustache, both in blue uniforms (I want to say French soldiers though the
movie was dubbed, not in French)

That would be Hector Servadac and his assistant Ben Zoof.

In a scene, two old men were playing chess in a castle where our main characters went to seek information. Old men were wise, maybe they were scientists. Cannons were fired or meteors were falling while they were playing. Gibraltar was taking place at some moment, our protagonists were sailing there.

Several of the characters sail around the sea in a ship searching for other survivors of the accident. At one point they find a solitary astronomer; at another they find a garrison of soldiers in a fortress on a fragment of Gibraltar, completely unaware of the nature of the catastrophe, and yes I believe two of them were playing chess.

More to the point, in 1979 an Australian company produced a 50-minute adaptation as part of their Children's Animated Classics: Off on a Comet. I've never seen it and have no idea how much of the original story was included. This is indexed in IMDB (that link). There was a CBS series a few years earlier "Famous Classic Tales" that apparently had an episode with this story. EDIT: It appears that the CBS series used the Australian production, and the 1979 date is for a home distribution by the Australian company.