How do I show my global Git configuration?

The shortest,

git config -l

shows all inherited values from: system, global and local

How do I edit my global Git configuration?

Short answer: git config --edit --global

To understand Git configuration, you should know that:

Git configuration variables can be stored at three different levels. Each level overrides values at the previous level.

1. System level (applied to every user on the system and all their repositories)

2. Global level (values specific personally to you, the user).

3. Repository level (specific to that single repository)

to view, git config --list --local
to set, git config --local core.ignorecase true (--local optional)
to edit repository config file, git config --edit --local (--local optional)

How do I view all settings?

Run git config --list, showing system, global, and (if inside a repository) local configs
Run git config --list --show-origin, also shows the origin file of each config item

How do I read one particular configuration?

Run git config user.name to get user.name, for example.
You may also specify options --system, --global, --local to read that value at a particular level.

Reference: 1.6 Getting Started - First-Time Git Setup

You can use:

git config --list

or look at your ~/.gitconfig file. The local configuration will be in your repository's .git/config file.

Use:

git config --list --show-origin

to see where that setting is defined (global, user, repo, etc...)

Git configuration variables can be stored at three different levels. Each level overrides values at the previous level.