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.
Swatragh GAC was created in 1946.
Faughanvale GAC was created in 1933.
Ballinascreen GAC was created in 1933.
Barna GAC was created in 1965.
Drum GAC was created in 1937.
An Riocht GAC was created in 1981.
Dungiven GAC was created in 1943.
Castledawson GAC was created in 1932.
Corofin GAC was created in 1884.
Craigbane GAC was created in 1972.
Clonduff GAC was created in 1887.