How much is a 1978 gold one dollar coin worth?
The U.S. did not produce any gold coins from 1933 until 1986 (bullion only) a dollar coin dated 1978 is an EISENHOWER dollar. It may have been gold plated or just toned a gold color but it's not gold. This piece has no numismatic value and is just a dollar, unless someone wants it.
You need to be more specific. There have not been any gold dollars released by the US Mint since 1889 for circulation (minus some minor collector editions before 1933) and there are no gold dollars dated 1978 struck by the US Mint. Instead you either have a gold plated 1978 dollar coin (which would just be worth $1) or you have a dollar coin issued by a different country or private mint.