Emerge git with the required USE flags:

emerge -aq dev-vcs/git curl gpg iconv nls pcre perl safe-directory webdav cgi cvs highlight keyring tk

Configure Apache vhosts (NO AUTH):

<VirtualHost *:443>
	  SSLEngine on
        <FilesMatch "\.(cgi|shtml|phtml|php)$">
          SSLOptions +StdEnvVars
        </FilesMatch>
        <Directory "/var/www/localhost/cgi-bin">
          SSLOptions +StdEnvVars
        </Directory>
          SSLCertificateFile /path/to/cert.crt
          SSLCertificateKeyFile /path/to/key.key
          ServerName git.example.com
          DocumentRoot /usr/share/gitweb
	  Options ExecCGI FollowSymLinks SymLinksIfOwnerMatch
	<Location />
	  SSLRequireSSL
	  Require all allowed
          RewriteOptions inherit
          AllowOverride All
	</Location>
        <Directory /usr/share/gitweb>
          Require all granted
          RewriteOptions inherit
          AllowOverride All
          Options ExecCGI FollowSymLinks SymLinksIfOwnerMatch
          <Files gitweb.cgi>
            SetHandler cgi-script
          </Files>
        </Directory>
	  DirectoryIndex gitweb.cgi
          SetEnv  GITWEB_CONFIG  /etc/gitweb.conf
        <IfModule headers_module>
          RequestHeader set X-HTTPS 1
        </IfModule>
</VirtualHost>

Basic /etc/gitweb.conf

$projectroot = '/path/to/gitroot';
@git_base_url_list = ( 'git@example.com', );
$site_name = "Smack my git up!";
$default_projects_order = "age";
$projects_list_group_categories = 1;
$feature{'blame'}{'default'} = [1];
$feature{'highlight'}{'default'} = [1];

Adjust git-daemon
Gitweb allows browsing repositories via HTTP, but if you will be pulling from your repositories using the git:// protocol, you'll also want to run git-daemon. On Gentoo, this is really easy, just edit /etc/conf.d/git-daemon as you see fit. eg:

GITDAEMON_OPTS="--syslog --enable=receive-pack --export-all"

This exports all repositories within the git root. It also allows pushing file to the server.

You will also need to ensure that any clients to connect to port tcp/9418 (default)

Setup the git user

useradd -m git -d /path/to/githome -s /usr/bin/git-shell
mkdir /path/to/githome/.ssh
cat /home/A_USER/.ssh/id_rsa.pub > /path/to/githome/.ssh/authorized_keys
chmod 700 /path/to/githome/.ssh
chmod 600 /path/to/githome/.ssh/authorized_keys
setfacl -PRdm u:apache:rwx /path/to/githome

Create an Empty Repository

su -s /bin/bash git
cd
mkdir name.git
cd name.git
git init --bare
echo "This is a test" > description
git remote add origin git@example.com:name.git

Set Configuration Options

Execute the following on a client (optional)
To see available options execute: git help --config

git config --global init.defaultBranch master
git config --global core.editor vim
git config --global alias.st status
git config --global alias.co checkout
git config --global alias.br branch
git config --global alias.up rebase
git config --global alias.ci commit
git config --global user.name "A_USER"
git config --global user.email A_USER@example.com
git config --global color.ui true
git config --global --add --bool push.autoSetupRemote true

Clone Repo

cd to local git
git clone git@example.com:name.git
cd name
touch README.md
git add README.md
git ci -m "Initial Setup"
git push
git br branch_name
git co branch_name
mkdir testdir
touch testdir/testfile
git add .
git ci -m "init"
git push # Pushes to your upstream branch
git push origin branch_name:master # Pushes to the master branch

Web Authentication

Note: I did not have any success using dbmmanage. Use htdbm instead.

Create the database file and add a user:
Do not store this in the root of web eg: /var/www/locahost/htdocs, but make sure apache can get to the file.

htdbm -cm -TGDBM <path/to/dbm_file> <user>

-c - create db
-m - use MD5 hashing
-T - set the format of the db file

Set the permission for apache:

chown apache: <path/to/dbm_file>
chmod 640 <path/to/dbm_file>

Add code to vhosts:
You can apply this to files, directories etc. Here I've done it for the whole subdomain. Replace the 3 highlighted italic lines above with the code below, excluding the start/end tags.

    <Location />
      AuthName "Private"
      AuthType Basic
      AuthBasicProvider dbm
      AuthDBMType GDBM
      AuthDBMUserFile "/path/to/dbm_file"
      Require valid-user 
      Require all denied
      RewriteOptions inherit
      AllowOverride All
    </Location>

Reload apache and browse to protected area and you should be prompted to authenticate.

A good explanation can be found here for the web config.