Is One Piece named after Gol D. Roger discovered it? We know that One Piece is the goal for Luffy to become pirate king, but I want to know about One Piece itself.

Did One Piece named after Gol d. Roger discovered it or its already One Piece when he discovered it?
I always believed that he said he left his treasure in "one piece" (which is a phrase that means intact/complete/undamaged/undivided).  As so little is known and that is the only decription, it became the defacto name.

This implies that the popular name was created after Gol's death.

However, the nature of the One Piece treasure and specifically what he meant is not fully clarified yet.  This means we don't know if he said "one piece" (intact) or "One Piece" (name for something or a play on words).

To fully answer this properly if this is a pun, you may need to find the original untranslated wording and someone who understands English and Japanese well.  Alternatively, wait until it is revealed (in several years hopefully).