If Adventuring Gear leaves the battlefield before its landfall ability resolves, the creature it was attached to at the time it left the battlefield gets +2/+2. If it wasn't attached to a creature at that time, nothing gets the bonus.

The landfall ability gives +2/+2 to the creature that's equipped by Adventuring Gear at the time that ability resolves. It doesn't matter what creature Adventuring Gear was attached to (if any) when it triggered. That bonus remains with that creature even if Adventuring Gear leaves the battlefield or becomes attached to a different creature later in the turn.

A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.

A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.

Whenever a land you control enters, each landfall ability of the permanents you control will trigger. You can put them   on the stack in any order. The last ability you put on the stack will be the first one to resolve (As a result, you can have those abilities resolve in the order of your choosing.).