I'm attempting a retranslation of Journey to the West that adheres more closely to the Chinese than translations I've found so far.
In the Jenner translation, this is left out.
In Anthony Yu's translation, it's translated as "We heard that", but this doesn't explain to me what the 蓋 means.
I'm guessing that the sense of the phrase is something like "Low key the news is", or "On the down low, hear that...".
The original text is here: https://www.gutenberg.org/cache/epub/23962/pg23962.html