If you need to update or add an email address to your GPG key, see Associating an email with your GPG key.
Commits and tags may contain several email addresses. For commits, there is the author — the person who wrote the code — and the committer — the person who added the commit to the tree. When signing a commit with Git, whether it be during a merge, cherry-pick, or normal git commit, the committer email address will be yours, even if the author email address isn't. Tags are more simple: The tagger email address is always the user who created the tag.
If you need to change your committer or tagger email address, see Setting your commit email address.