the Jester said:
The 'astral gestation period' issue is the one that could win me over. Must mull.
Or if you still don't like eggs but want to come up with an explanation otherwise:
A rapid-gestation spell or magic item could be a Githyanki secret, which twists the passage of time on the Astral or the development of the mother and child to develop normally (subjective to the Githyanki perception).
Or, the Githyanki could have secret "nursery demiplanes" created as little pocket realities with the exact conditions carefully engineered to resemble the Astral in many ways but allow for normal progression of biological time, where Githyanki are concieved, gestated, born and raised, and they may visit the Astral to see their families and for training and familiarization for some time (or possibly go there temporarily to be physically born before being taken back to their nursery plane, if their actual plane of physical birth determines their native plane), but eventually they have to return to their "nursery" plane to mature. These little enclaves would doubtless be one of the tightest secrets of the Githyanki, known only to their race, since divulging their location could endanger their entire race.
After all, if a pregnancy on the Astral might last a millennium, growing to adulthood could take tens of thousands of years, which is probably longer than the lifespan of the Githyanki race to-date by all indications, they would have to have some way to grow to adulthood in a normal time span or they could be wiped out, with all the losses they doubtless take rapidly outpacing their ability to reproduce.
They might even create the whole "laying eggs" story as disinformation, complete with backstory about Illithid modifications to make people think that's how they breed (with maybe some creative use of Illusion and Transmutation spells to back up the illusion), so nobody goes looking for the place where Githyanki are born and mature before becoming nigh-immortal warriors of the Astral.