Add API docs for commit's GPG signature endpoint

changelogs/unreleased/49502-gpg-signature-api-endpoint.yml

+---
+title: Add API endpoint to get a commit's GPG signature
+merge_request: 25032
+author:
+type: added

doc/api/commits.md

@@ -656,6 +656,46 @@ Example response:
 ]
 ```
 
+## Get GPG signature of a commit
+
+Get the [GPG signature from a commit](../user/project/repository/gpg_signed_commits/index.md),
+if it is signed. For unsigned commits, it results in a 404 response.
+
+```
+GET /projects/:id/repository/commits/:sha/signature
+```
+
+Parameters:
+
+| Attribute | Type | Required | Description |
+| --------- | ---- | -------- | ----------- |
+| `id`      | integer/string | yes | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user
+| `sha` | string | yes | The commit hash or name of a repository branch or tag |
+
+```bash
+curl --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/projects/1/repository/commits/da738facbc19eb2fc2cef57c49be0e6038570352/signature"
+```
+
+Example response if commit is signed:
+
+```json
+{
+  "gpg_key_id": 1,
+  "gpg_key_primary_keyid": "8254AAB3FBD54AC9",
+  "gpg_key_user_name": "John Doe",
+  "gpg_key_user_email": "johndoe@example.com",
+  "verification_status": "verified",
+  "gpg_key_subkey_id": null
+}
+```
+
+Example response if commit is unsigned:
+```json
+{
+  "message": "404 GPG Signature Not Found"
+}
+```
+
 [ce-6096]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6096 "Multi-file commit"
 [ce-8047]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8047
 [ce-15026]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15026

doc/user/project/repository/gpg_signed_commits/index.md

@@ -266,3 +266,7 @@ To remove a GPG key from your account:
 
 You can configure your project to reject commits that aren't GPG-signed
 via [push rules](../../../../push_rules/push_rules.md).
+
+## GPG signing API
+
+Learn how to [get the GPG signature from a commit via API](../../../../api/commits.md#get-gpg-signature-of-a-commit).