A gravity bubble was a form of technology that allowed an object to beat the boundaries of gravity and travel in space.
Milo and Frank Shakespeare used gravity bubble technology to transport a Nukaryote to the fathership in space. (COMIC: Doctor Who and the Nightmare Game)
Edwin Bracewell applied this technology to Spitfires, allowing the pilots to travel around the orbit of planet Earth and attack the Dalek Saucer. (TV: Victory of the Daleks) Though this advanced technology was to be dismantled to avoid altering history, it was used again when the Doctor summoned "Danny Boy" to Demon's Run, where he destroyed the facility's communications array, then returned to his own time. (TV: A Good Man Goes to War)
Silurians used this technology to transport themselves from their civilisation to the surface of Earth. (TV: Cold Blood)
Other uses
Just before the Master's TARDIS materialise into the Doctor's one, Adric and the Fourth Doctor guess a gravity bubble is responsible for the instrumentation failure of the TARDIS. (TV: Logopolis)