I'm new to gzdoom and I'm having the same problems with other games, such as The Adventures of Square (https://bigbrikgames.itch.io/square). I can play the games but permanently have these graphics errors. I use MX-15 Debian Jessie 32 bit with Open source drivers Nouveau from 2016 for a GeForce GT 710. Since younger versions of gzdoom do not work for me I use version 4.1.3
Kind regards