Improve abstract attribute usage in install-rhdh-osd-gcp

- Move abstract text to :abstract: attribute definition
- Reference with {abstract} in [role="_abstract"] section (DRY)
- Remove extra blank line after content type metadata

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: themr0c

titles/install-rhdh-osd-gcp/master.adoc

@@ -1,19 +1,17 @@
 :_mod-docs-content-type: ASSEMBLY
 
-
 include::artifacts/attributes.adoc[]
 :title: Installing {product} on {osd-short} on {gcp-brand-name}
 :subtitle: Running {product} on {osd-brand-name} by using either the Operator or Helm chart
-:abstract: Platform administrators can configure roles, permissions, and other settings to enable other authorized users to deploy a {product} ({product-very-short}) instance on {osd-brand-name} on {gcp-brand-name} ({gcp-short}) using either the Operator or Helm chart.
-
+:abstract: Deploy {product-short} on {osd-short} on {gcp-short} to provide a unified developer experience platform using either the Operator for centralized management or Helm for flexible configuration.
 :context: title-install-rhdh-osd-gcp
 
 [id="{context}"]
 = {title}
 :imagesdir: images
 
 [role="_abstract"]
-Deploy {product-short} on {osd-short} on {gcp-short} to provide a unified developer experience platform using either the Operator for centralized management or Helm for flexible configuration.
+{abstract}
 
 [IMPORTANT]
 ====