Add the Namespace

"using System.EnterpriseServices.Internal;"

String SourceLocation = ConfigurationManager.AppSettings["SourceDLL"].ToString();            
if (Directory.Exists(SourceLocation))
{
     foreach(String sFileName in Directory.GetFiles(SourceLocation))
     {
          FileInfo FileName = new FileInfo(sFileName);

          Publish oPublish = new Publish();

          //To Remove the DLL in assembly
          oPublish.GacRemove(SourceLocation + "\\" + FileName.Name.ToString());

          //To Install the DLL in assembly
          oPublish.GacInstall(SourceLocation + "\\" + FileName.Name.ToString());
     }
}

There is no need to remove the dll seperately as mentioned in the code, it can be automatically overwritten.