Maintaining cache coherency is an important part of a RAC activity. Cache coherency is the technique of keeping multiple copies of a block consistent between different Oracle instances. Global Cache Service is a part of it. GCS implements cache coherency by using what is called the Cache Fusion algorithm. GCS is not a process, it's a service which manages the information in the the GlobalResource Directory (GRD). Global Cache Service Processes (LMSn) is a RAC background process. With other RAC background processes it manages the global resources. LMSn manages the GCS. it's a part of it.