You find the 'Beta 1' iso files at the ISO testing tracker.
Ubuntu Gnome is 'merged' into standard Ubuntu in Bionic (to be released as 18.04 LTS).
There is also another reason why a Bionic Beta 1 iso file is missing: That flavour of Ubuntu is not taking part in the beta 1 testing.
But all flavours have daily built iso files at the ISO testing tracker.
And standard Ubuntu Desktop is not taking part in the beta 1 testing. So this (among the daily built iso files) is where to find its iso file (at the correct time interval corresponding to 'Beta 1', after that the daily built iso files are updated with new versions of program packages in the development process).