(GONRG-1552) GCP: update documentation

Type of change

• Bug Fix
• Feature

Does this introduce a change in the core logic?

• [NO]

Does this introduce a change in the cloud provider implementation, if so which cloud?

• AWS
• Azure
• GCP
• IBM

Does this introduce a breaking change?

• [NO]

What is the current behavior?

Outdated documentation

What is the new/expected behavior?

Actual documentation

Have you added/updated Unit Tests and Integration Tests?

• [NO]

Any other useful information

Changes:

• README files, images