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.
Claudy GAC was created in 1934.
Portaferry GAC was created in 1912.
Castlewellan GAC was created in 1905.
Ardmore GAC was created in 1938.
Kilrea GAC was created in 1956.
Banagher GAC was created in 1965.
Bredagh GAC was created in 1973.
Bellaghy GAC was created in 1939.
Lissan GAC was created in 1910.
Caltra GAC was created in 1899.
An Lúb GAC was created in 1933.