gpMods.dll: Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information
I search for hours for this one, apparently it can be caused by lots of things. In my case, I got the error deploying a project from a dev server, where it worked just fine, to a production server.
I had installed the .NET 3.5 framework on my dev box and was using LINQ.
Installing 3.5 on the production box fixed it.