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By default the GAC folder in Win­dows is located at %windir%\assembly, you can find all the reg­is­tered DLLs in that folder. Whilst you can open it in win­dows explorer and view it like any other folder, it is a some­what spe­cial and allows you to have dif­fer­ent ver­sions of the same DLL reg­is­tered in the GAC. 

Open up DOS prompt and nav­i­gate to the GAC folder, for instance:

and you can see that the GAC is actu­ally a folder inside the assem­bly folder, and drilling a lit­tle deeper reveals that each DLL has its own folder which con­tains all the reg­is­tered ver­sions, each as a folder that con­tains the actual DLL inside:

.Net 4

For .Net 4, the GAC loca­tion is now %windir%\Microsoft.Net\assembly.

Global Assem­bly Cache Tool

You can use gacutil.exe from the com­mand line to view (e.g. gacu­til /l), add (e.g. gacu­til /i SomeAssembly.dll) or remove (e.g. gacu­til /u Some­Assem­bly) the con­tents of the GAC, this of course, sup­ports mul­ti­ple ver­sions of the same assem­bly too.