Fix typo: GCP -> GPG

[beltphed]

Small typo, probably an autocorrect induced error. The text says "GCP" but the steps reference importing a GPG key.

Description

Checks

Docs building

• [x] Compiles without warnings.

Code formatting and web optimization

• [x] Uses three spaces indentation.
• [x] Adds or updates meta descriptions accordingly.
• [x] Updates the redirects.js script if necessary (check this guide).

Writing style

• [x] Uses present tense, active voice, and semi-formal registry.
• [x] Uses short, simple sentences.
• [x] Uses bold for user interface elements, italics for key terms or emphasis, and code font for Bash commands, file names, REST paths, and code.