GCP is important because it underpins the European Directive and clinical trial legislation in EU and other countries. Moreover, it is now much more widely adopted in countries in addition to the EU, USA and Japan and is now almost universally recognised as the standard for doing clinical research. Another aspect of GCP is to provide standardised guidance for quality standards and the content and structure of regulatory submission dossiers.
However, do not think that a good working knowledge of GCP iss all you need. Although local clinical trial legislation is largely based on GCP there are often differences that you need to be aware of. Being in breach of GCP is bad but breaking the law has much more far-reaching consequences. Do you where UK clinical trials legislation differs from GCP???