any recommendations on what to use to have the door gently close?
I don't know what type of hinges are on the door, the door is between the garage and house entrance.
People are going in & out all day long and all we hear is the door slamming shut....

You need a "door closer"

A mechanical /hydraulic device that controls the rate of closure.....usually top mounted......like any commercial door

Here's an example .....you need to match the general weight of the door to the specs.......

http://www.acehardware.com/product/index.jsp?productId=1297983

"...measure once.....cut twice....throw that one away and cut a new one...."