Is there a GUI version of nmap available?

Zenmap

You can use zenmap, it is in the repository

sudo apt-get install zenmap

Here is its website I think it is the official GUI as the site is nmap.org.

Umit

If you don't like that one try umit, it is in the repository as well

sudo apt-get install umit

This one did not have its own website, here is its source forge page

nmapsi4

Finally there is nmapsi4 too, this seems the best looking one, if you are in to that

sudo apt-get install nmapsi4

Again no website, but here is its launchpad page

Others

There are at least two more available, including a python one and a web interface but this is getting a bit long, to find them yourself, run this command

apt search nmap