For the whole Luke/Leia thing I do have the option for Gabriel to attempt to flirt with Lucifer, because writing that awkward scene amuses me to no end, especially since in this scenario one of them is aware of their relationship
Once more @avidreader is absolutely correct. Technically there is nothing forbidding it, but the offspring of angels are not allowed to be raised by their parents and going out of their way to see their offspring tends to be inconvenient and awkward as there's no strong familial bond.
You were right on the nose
There's nothing against having angel children, and Gabriel does have another parent, so someone else has to know for sure.
How many, who, and what they may do with that knowledge.... well...