To register an assembly in the GAC, you must first copy the dll to the c:/Program Files/Microsoft Visual Studio 8/SDK/v2.0 (or related software version folder)
Then, run the command prompt and change the active directory to the one listed above.
Type: gacutil /I [dllfilename].dll
then type: sn -T [dllfilename].dll to get the public key token to use when registering in the machine config. Then, under system.web --> compilation --> assemblies -->, type
<add assembly="SqlHelper, Version=1.0.0.0, Culture=neutral, PublicKeyToken=cae4b5dbd832c614"/>
where the assembly is the dll name and publickeytoken is the result of the above token command