Does myriapod include all arthropods gastropods and brachiopods?

No, myriapods are a subphylum of arthrpods, and include the centipedes, millipedes and their ilk. It would be better to say arthropoda includes all myriapods.

Gastropods are separate, in a different phylum - Mollusca (the molluscs), brachiopods likewise have their own phylum.