If you get this message in the task list (even though normally you will be able to build without an error), double click on one of the errors to find the form that it is linked to (the task list normally will not tell you). Exclude that project from the form. Some or all of the messages will disappear from the task list. When all of them are gone, re-include each of the forms to the project.
If that doesn't work, check to see if you have “copy local” set to true for any assemblies that are referenced, that are also in the GAC. Unless you need to ensure this local copy for some reason (usually for deployment reasons), set this to false.
Note: you can still successfully build a project that has these 'errors' in the task list.