I'm trying to deploy GitLab on Kubernetes using minikube through this tutorial, but I don't know what values to put in the fields global.hosts.domain, global.hosts.externalIP and certmanager-issuer.email.

For the domain, you can likely use whatever you'd like, just be aware that when gitlab generates links that are designed to point to itself they won't resolve. You can work-around that with something like dnsmasq or editing /etc/hosts, if it's important to you

For the externalIP, that will be what minikube ip emits, and is the IP through which you will communicate with gitlab (since you will not be able to use the Pod's IP addresses outside of minikube). If gitlab does not use a Service of type NodePort, you're in for some more hoop-jumping to expose those ports via minikube's IP

The certmanager-issuer.email you can just forget about, because it 100% will not issue you a Let's Encrypt cert running on minikube unless they have fixed cermanager to use the dns01 protocol. In order for Let's Encrypt to issue you a cert, they have to connect to the webserver for which they are issuing the cert, and (as you might guess) they will not be able to connect to your minikube IP. If you want to experience SSL on your gitlab instance, then issue the instance a self-signed cert and call it a draw.

The tutorial is very poor in explanations.

That's because what you are trying to do is perilous; minikube is not designed to run an entire gitlab instance, for the above and tens of other reasons. Google Cloud Platform offers generous credits to kick the tires on kubernetes, and will almost certainly have all the things you would need to make that stuff work.