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Gravity and vacuum are not mutually exclusive - you always have to deal with gravity forces, although they become negligible pretty quickly when you get into and then leave orbits.
As to the specific claim, I suspect that the experiments they are currently doing (in vacuum chambers on earth) have gotten to the point that they are measuring the propulsion system producing more thrust than it's own weight (T/W >1), which would technically be enough thrust to overcome gravity. Even if it wasn't practically useful for actually getting to orbit, that amount of thrust on a reactionless motor would be incredible, and would totally unlock the solar system for us.
they become negligible pretty quickly when you get into and then leave orbits.
You're not wrong, but it's worth noting that in low earth orbit (e.g. typical ISS orbiting altitude) earth's gravity is still 90% as strong as at sea level. Astronauts on the ISS are weightless not because they escaped earth's gravity, but because they're continuously falling.
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If I remember right that is the slang term for a device used to simulated high force situations astronauts are expected to experience. So named for the frequent effect
It's so much more interesting than the parent comment makes it out to be, though. In case you didn't already read anything about it, it's an ordinary (on the outside) jet that flies in parabolic arcs which at a certain point lets the passengers experience microgravity and float around the modified cabin for a minute or so at a time. Anyone can go for a ride last I checked... if you have several thousand dollars laying around, anyway.
Much of the same could be said of the EmDrive. This is one of those extraordinary claims which requires extraordinary evidence, especially given the recent history of the topic.