The official answer is %windir%\assembly
But that is just the registered assemblies. Don't do anything in the folder manually, use the API or the gacutil tool. If you are looking for the big list of files it is probably at the subdirectories of %windir%\assembly\GAC_MSIL
Dunloy GAC was created in 1908.
Castlewellan GAC was created in 1905.
Kilrea GAC was created in 1956.
GAC Toyota was created in 2004.
The population of GAC Toyota is 5,600.
Greenlough GAC was created in 1939.
Longstone GAC was created in 1945.
Portaferry GAC was created in 1912.
Ardmore GAC was created in 1938.
Banagher GAC was created in 1965.
Bredagh GAC was created in 1973.
Bellaghy GAC was created in 1939.