Reference the system.EnterpriseServicess assembly to your project.

using System.EnterpriseServices.Internal;

public void Update(DirectoryInfo folder_GAC)
{
  //check if folder containing new dll files exists
  if(folder_GAC.exists){
    //add each dll found in this folder to the Global Assembly Cache
    foreach (FileInfo dll in folder_GAC.GetFiles("*.dll")) 
    {
      //instantiate a new Publich object and install the dll
      Publish objpublish = new Publish();
      objpublish.GacInstall(dll.FullName);
    }    
  }
}

remove an assembly from the GAC can be done by using the GacRemove(string name) method.

more information:
http://msdn.microsoft.com/en-us/library ... blish.aspx