Yes, there are countless forgeries of the most varied quality: from clumsy copper alloys that bear little resemblance to real coins to perfectly imitated wolfram forgeries in coins and bars that no precious metals expert could recognise without suitable testing equipment. See photos of gold fakes in our gallery.
The most common types of counterfeiting are highway gold, under-alloying and over-alloying, cores made of foreign metal and minting/stamp counterfeiting.
You can find more information here.