No Proof $20 gold pieces are known from 1851-1857 except for a single 1854 rumored to exist in a Proof Set in Bremen, Germany. However, no one (to our knowledge) has seen this coin and the only reason we list it here is because of Walter Breen's citation. He also lists an example in Morgenthau 6/1940:5, which is also untraced and unconfirmed.
Sources and/or recommended reading:
Breen, Walter. Walter Breen's Complete Encyclopedia of U.S. and Colonial Coins, New York: F.C.I. Press, Inc., 1988