SETUP:

The setup process works like this:
Decide where you want the base directory for gitea
  for example: /srv/services/gitea
# mkdir -p /srv/services
Make the directory itself writable
# chmod o+w /srv/services
Decide the user/group you want to own the directory and service and change the container
$ sed -i 's/1009/<insert-number>/g' gitea-container.scm

for running the service:
see included gitea.service

for upgrades:
# guix pull
# systemctl restart gitea.service