You might be
A sewer cover is my cousin, we share many traits
They're both flat and metal, roughly the same shape.
People who have me, often lock me away
Misers hiding money away in their houses, or locking money in a safe.
I am two-faced but bare only one
A coin has two sides, but usually only one bears a bare face.
People do anything to get me, sometimes using a gun
I have the most power when given away
Money doesn't have much power in itself, unless it can be spent on something.
(Thanks @LifelongScholar for help with this clue!)
And I am often on display
Maybe in a coin collector's album.