Finally realized what I was doing wrong. Su -c requires quotes around the command part.

linux-game-manager.sh

@@ -518,8 +518,7 @@ case "${game}" in
             if command -v yay &> /dev/null ; then
                 yay -Sy --noconfirm --sudoloop freedoom gzdoom
             elif command -v slapt-get &> /dev/null ; then
-                su -c slapt-get -i gzdoom
-                su -c slapt-get -i freedoom
+                su -c 'slapt-get -i freedoom gzdoom'
             else
                 dialog --backtitle "Linux Game Manager" --msgbox "No supported package managers found. Please install the freedoom and gzdoom packages manually." -1 -1
                 exit 0