Can I uninstall UFW completely?

Yes, you can remove or disable ufw and/or gufw without any problem. It will not affect your iptables configuration. UFW (Uncomplicated Firewall) was simply developed to ease some configurations done with iptables.

Removing it will not affect your iptables configuration.

To disable UFW you can type the following:

sudo ufw disable

To remove it you can type the following:

sudo apt-get remove ufw

To purge it (In those cases where you really hate ufw or wasting space) you can type the following:

sudo apt-get purge ufw

The only thing you can not do is remove iptables. This is because it is a dependency package of ufw, therefore ufw depends on iptables (Not the other way around ^^).

I had the same problem. After command

iptables -L

I saw remians of ufw.

Just use two commands:

iptables -F

iptables -Z

-F is for flush

-Z is for zero chains