Easiest is to have a path under main domain instead of subdomain. Try adding the following /etc/apache2/conf-available/my-gitea.conf
:
<Location /gitea/>
ProxyPass http://localhost:3000/
</Location>
Then
sudo a2enconf my-gitea
systemctl reload apache2
See other configuration files in /etc/apache2/conf-available/
provided by FreedomBox.