('US') She shifted 'into' low/high 'gear'. = ('Brit') She changed 'into' bottom/top 'gear'. [=she changed to a gear that allows for a slower/faster rate of speed]
He put the car 'in/into gear' [=he moved the lever that controls the car's gears into the position that allows the car to begin moving] and drove away.
('US, informal + impolite') He angrily told him to 'get his ass in gear'. [=to get going, to start moving or doing something much more quickly]
(..) Hold on a second. Hey, Wainfleet! Get it done! - We bounce at zero-nine. - Yeah, I'm on it, capitaine! Vine strike's still loose. You guys are packing some heavy gear. Watch it! Yeah, that's 'cause we're not the only thing flying around out there. Or the biggest. I thought you'd never ask. (..)2009 Avatar
(..) I do not look down on niggers, kikes, wops or greasers. Here, you are all equally worthless. And my orders are to weed out all non-hackers... ...who do not pack the gear to serve in my beloved Corps. Do you maggots understand that?. Sir, yes, sir. Bullshit. I can't hear you. Sir, yes, sir! (..)1987 Full Metal Jacket
(..) Can't say I liked it much. Lots of bugs and too dangerous. As it happens, my present duties keep me where I belong... ...in the rear with the gear. Tet. The year of the monkey. Vietnamese Lunar New Year's Eve. Down in Dogpatch, the gooks are shooting off fireworks to celebrate. (..)1987 Full Metal Jacket