GCP stands for Global Company Prefix, which is a unique identifier for a company that forms part of each GTIN. A given company can own more than one GCP. When you buy a batch of GTINs from GS1, they all come with the same GCP. So when you buy a new batch, you get another new GCP. Polytag uses this information as part of its security measures to ensure that only your account contains your GTINs.
See this example of a GCP.
For further reading on GCP's, please visit this part of the GS1 website