You can quasi-use the GUI if you're ok with manually editing the settings.php file. Details:
In "/var/www/html/admin/settings.php", on line 473, change "if ($DHCP) {" to "if (true) {". This will enable the leases section of the DHCP GUI. Then, put your static DHCP entries, if you have any, in:
/etc/dnsmasq.d/04-pihole-static-dhcp.conf
Then, store your dhcp.lease file in /etc/pihole/dhcp.leases. I use this line in my dnsmasq.conf file:
dhcp-leasefile=/etc/pihole/dhcp.leases
This will allow you to see any static reservations you have configured, and, it will also display the current leases.