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So the big FAQ.

'BATTLE BROTHERS
All of the units in each Detachment in your Battle-forged army must have at least one Faction keyword in common. In addition, this keyword cannot
be Chaos, Imperium, Aeldari, Ynnari or Tyranids, unless the Detachment in question is a Fortification Network. This has no effect on your
Army Faction.'

Does this mean the GSC cultist I have been painting the last 2 months cannot support my tyranids?

No.
It means you can't mix them in one detachment.

"This has no effect on your Army Faction."

It just means you can't include Cult Leman Russes in the same Detachment as Hive Tyrants, for example.

You need to isolate your GSC and Tyranids into two different detachments.

Also keep in mind this is a Beta rule.

Actually, I think you might be right...