Validation rules are what GCC should use to ensure that opportunities meet the standards for the Annual Revenue field. Validation rules are a type of metadata that define a condition and an error message to display when the condition is not met. Validation rules can be used to enforce data quality and integrity by preventing users from saving records that do not meet the specified criteria. For example, a validation rule could be set up on the Opportunity object to check that the Annual Revenue field is a positive amount and does not exceed
$100. Assignment rules are a type of metadata that define how records are assigned to users or queues based on certain criteria. Assignment rules can be used to automate business processes by distributing records to the appropriate owners or teams. For example, an assignment rule could be set up on the Lead object to assign leads to sales reps based on their territory. Default values are a type of metadata that define a value that is automatically populated in a field when a new record is created, unless the user enters a different value.
Default values can be used to save time and ensure data consistency by prefilling fields with common or expected values. For example, a default value could be set up on the Lead Source field to indicate the most frequent source of leads, such as Web or Phone.

The tool that GCC should use to build an integration between its financial system and its Salesforce environment is MuleSoft. MuleSoft is a platform that enables companies to connect any application, data, or device using APIs. MuleSoft helps GCC to create and manage integrations between Salesforce and other systems, such as its financial system, without writing any code. MuleSoft also provides prebuilt connectors, templates, and best practices to speed up the integration process