If you do not specify it, there is no outgoing email enabled. You have to enable it by specifying some settings. The following is the install command if you set the options right away:

$ helm upgrade --install gitlab gitlab/gitlab 
 --timeout 600 
 --set global.hosts.domain=home.joustie.nl 
 --set global.hosts.externalIP=<your external ip> 
 --set [email protected] 
 --set global.smtp.enabled=true 
 --set global.smtp.address=smtp.xs4all.nl 
 --set global.smtp.port=25

Also, make sure there is no firewall preventing traffic in between. Clusters on Google Kubernetes Engine (GKE) have their SMTP ports blocked by default.