The most important question in my mind is not WHY the Gbaba are doing what they do, but how they find their victims.
What attracts them? And what can you do to avoid being found?
- Did a scout ship enter one of their systems? Then you should not send any scouts out to explore the galaxy until you have a big fleet.
- Is it simple radio wave emissions? In which case there was a listening post somwhere within 500 LY of earth and it's colonies (ca 500 years from the invention of radio to first contact) and likely it was at least 400 LY from earth. Assuming an even distribution of listening posts in the entire galaxy (worst case) Safehold has a maximum of 500 years from reintroduction of space travel until the war.
- Is it FTL drive emissions? Or hypercom emissions? The first is unlikely since they would be able to track the colo fleet to Safehold and the second is also not very likely because Crestwell's Star didn't have a hypercom. But just to be sure you should avoid both techs until you are ready.
- Are they simply sending small exterminator fleets to random systems? Then you have to pray for good luck.
- Or maybe it's something entirely different. Maybe they are telepaths and they consider every species which cannot communicate by telepathy as not sentient.
From the answer to this question you can infer reasons for their behaviour:
- They think the were attacked by a scout. The whole invasion is an automated "seek&destroy" process.
- They met another race and barely survived. Deep seated xenophobia
- Hyperspace/The Galaxy/The Status Quo is holy and humans profane/endanger it by their very existance
- They are allergic to radio waves or at least a certain spectrum of it
- Their hive-mind works by hyperspace connections and human hyperspace traffic within 500 LY gives them "headaches". Alternatively human brainwaves interfere with their telepathy
So I think we first have to find out what the humans and the other species did to attract them.
PS: And by "find out" I mean speculate since we won't know anything about it until we read it in one of the upcoming books