Hello, with:
--8<---------------cut here---------------start------------->8---
(service home-git-service-type
 (home-git-configuration
  (options '((user (name "foo")
                   (email "foo@bar.com"))))))
--8<---------------cut here---------------end--------------->8---

We'll have git in the home profile, and GIT_CONFIG_SYSTEM pointing to a
file contains:
--8<---------------cut here---------------start------------->8---
[user]
	name = "foo"
	email = "foo@bar.com"
--8<---------------cut here---------------end--------------->8---

Which set the system-level options for git.