The GB is stuck on MATTHEW. But if you go to LUKE 21:32 - (you'll see it's the same as matthew, says "this generation will be no means pass away until all things occur". You'll see it speaks of moon and sun darkened in vs 25 to 28 just as in matthew in the same spot.
Now if you go back further to Luke 21:20 STOP THERE! "When you see Jerusalem surrounded by encamped armies, then know that the desolating of her has drawn near."
That's not in Matthew, in Matthew it says when you catch sight of the disgusting thing causing desolation.
The reason GB doesn't use Luke, is because it PROVES that Jesus in that particular instance, was speaking to the LITERAL generation, who LITERALLY saw the surrounding armies encamp Jerusalem and destroy it!
There, that's it. GB also probably knows if trying to apply a 'modern day' fulfillment, it 'would' be tied to Jesus presence, and since 1914 was wrong, they will do anything possible to apply generation to 1914, because they've made such a big deal about it, it can't be dropped