If I was to put a (-) charged plate on top of a lot of (-) gas particles, and then surronded the edge of the plate (extending down to the ground) with a magnetic field, wouldn't the plate float due to the like charges wanting to repele from each other?

yes. although a negatively charged gas would not stay negatively charged for long as it would lose charge to its container.

why would it lose its charge if its container has the same charge?

By depositing the charge on the container wall it reaches a lower energy level as like charges repel. the negative charge of the container would not repel the gas as the electric field strength inside a conductor is zero.