Okay, this is silly because I keep forgetting the exact syntax, but this is incredibly useful whenever writing any code for SharePoint.
So I'm putting this on my blog for future copy paste pleasure -
Just add the following to PostBuild Events -
"C:\Program Files\Microsoft Visual Studio 8\SDK\v2.0\Bin\gacutil.exe" /i $(TargetPath)
By doing so, everytime you build your class library, it will automatically be added to the GAC for you.
One less thing to worry about - bak2wurk!