bottom, top (both BrE)
She was driving along in top gear.

high, low
first, second, etc.
reverse

engage, select
Engage first gear and move off.

change (esp. BrE), shift (AmE), switch (usually figurative)
It's difficult to steer and change/shift gear at the same time.
He found it hard to switch gears when he retired.

change into (BrE), move into, put sth into, shift into (AmE), slam sth into, slip sth into
She put the car into first gear and drove off.

move up, step up (often figurative)
Coming out of the final bend, the runner stepped up a gear to overtake the rest of the pack.

crash (BrE), grind
He was crashing the gears because he was so nervous.

change
She made a smooth gear change.

lever, shift (AmE), stick (usually gearstick)

in (a/the) gear
I was driving along in third gear.
Some drivers leave the car in gear when parking on hills.

out of gear
Leave the car out of gear.

camping, climbing, fishing, hiking, riding, running, sports, swimming
breathing (esp. BrE)
firemen in breathing gear

protective
The hot weather makes it hard for soldiers in protective gear.

landing
the plane's landing gear

battle, combat, riot
outdoor
designer
She was the only one wearing expensive designer gear.

be dressed in, have on, wear
She had her running gear on.

put on, take off

in … gear
a group of men in combat gear