The term originates from hackey-sack. A gravity check is when an easily playable sack hits the ground only because it was coming down equidistant between two or more players and each player didn't make an attempt because they thought someone else would play it.
The term can also be used in business to describe a situation where nobody on the team works on a task because they think someone else must already be working on it.
Your boss emails the whole team, "Gravity check. Earlier I sent an email to all of you asking that one of you please call the client today. Did any of you actually call the client?"
A dead drop is a location used to secretly pass items between two people, without requiring them to meet.
Spies have been known to use dead drops, using various techniques to hide the items and to signal that the drop has been made. Drug dealers use this as well.
A common place is to duct tape the item to the underside of a toilet cover in a public restroom.
A signal is sometimes used as well to indicate there is something to pickup, like a pile of stones in a certain formation. After checking the drop the signal is removed.
"The dead drop is the restroom toilet at the Gulf station at the corner of Commercial Ave. and Georges Rd."