git-gui: add Spanish translation

[kant]

• [x] The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• [x] The page(s) have at most 8 examples.
• [x] The page description(s) have links to documentation or a homepage.
• [x] The page(s) follow the content guidelines.
• [x] The PR title conforms to the recommended templates.
• Version of the command being documented (if known):

[tldr-bot]

Hello! I've noticed something unusual when checking this PR:

• The page pages.es/common/git-gui.md is outdated, based on the command contents itself.

Is this intended? If so, just ignore this comment. Otherwise, please double-check the commits.